JIMMY CARTER was rounding the edge of Hains Point last night, two Secret Service agents padding behind him, the lights of the Capitol shining before him. He was keeping a good 8 1/2-minute-mile pace in the chilly darkness. He talked easily to a reporter he'd invited along for the jog. "I don't miss Washington," he said, his brow broken out in a healthy sweat. "But I do miss being president."

Carter had just spent a long day on his old turf, vigorously promoting his new book, "Keeping Faith." At 6:30 last night, dressed in a windbreaker and blue running shorts, he settled into the blue plush seat of a limousine outside Jackson Place, the house on Lafayette Square where former presidents stay when they're in town. The car drove down 17th Street, past the Executive Office Building and then into view of the Jefferson Memorial. The monument's mirror image glimmered in the Tidal Basin. "Such a pretty place," he said, looking toward it. "Rosalynn and I used to sit on the Truman Balcony and watch the planes land right behind there."

Once at Hains Point, he set the pace. The agents stayed several paces behind, although every time there was a rustle in the bushes, one ran ahead to check things out. Carter mentioned the day's press conference. "How was it?" he was asked. "Oh, I don't know," he said casually. "It's hard to tell when you're answering the questions."

He talked about places he'd run around the world, and his continued friendships with former advisers Hamilton Jordan, Jody Powell and Jerry Rafshoon. He said he's closest to Powell, and that Jordan became much more deferential after Carter became president. But Rafshoon, he said, treats him the same way he always had. When he was governor of Georgia, Carter said, someone once asked Rafshoon if he always called Carter "Jimmy." Not always, Carter recalled that Rafshoon replied. Sometimes he referred to him as " ---- you, Governor."

He rounded the tip of the point, the 3.5 miles nearly half over. "If I were president," he said, "we'd have problems with the deficit and unemployment. But they wouldn't be as bad as they are now." And he mentioned his 1979 trip to Egypt to work out the final points of the Camp David accords, saying he settled things with then-Egyptian President Anwar Sadat in the first 20 minutes, so "we had a good time for the next two days, just fooling around with our families."

He recalled his first transition meeting with Ronald Reagan. "It was one of the strangest experiences of my life," he said. A little later, he talked of his daughter Amy, saying that she's "at that age" where she and his wife, Rosalynn, argue about how her hair should be combed and what dress she should wear. "I try to stay out of it," he said.

He ran on through the darkness.

Eight hours earlier, Jimmy Carter had settled into a chair in front of an oil painting of Andrew Jackson hung in an upstairs room at Jackson Place. He was dressed in a dark suit and striped tie, his back to the White House. "Most people, at least certainly around Washington," he said, "have always felt that I was a man of mystery, that there was something hidden within my consciousness, or within my psyche, that had to be revealed in some fashion -- that there must be something here that's being concealed from us about inner feelings and motivations. I consider myself to be a very simple and open person."

But to many of his friends and former advisers, Carter remains the puzzle he always was. Certainly his new book, 596 generally impersonal pages, hasn't revealed any clues; if anything, it has baffled and disappointed some of those who knew him.

"I don't know what else I have to reveal," Carter said. "I think the book does explain my motivations, my ideals, my background . . . You know, Henry Kissinger has written 3,000 pages already -- and he hasn't even gotten to the Ford administration."

Carter was all over town yesterday, as intense and earnest an author as there ever was on a tightly packed book promotion tour. He attended a breakfast with reporters, gave numerous interviews, held a press conference and, for more than an hour in the morning, sat down for a casual talk. He spoke about Washington, the election, the Democrats, his disappointments, his family and his marriage. He seemed relaxed, and quickly pulled out the familiar, self-deprecating charm. Told by his interviewer that they had met once before at the White House, Carter smiled and replied:

"Thanks for remembering."

Asked who is the better politician -- himself or Ronald Reagan -- Carter replied swiftly: "In 1980, he was."

But overall?

Carter paused. "That remains to be seen," he said. "The reason I hesitate is that I don't know. At this point in Reagan's term, he's probably less popular than I was. I had a tendency then and now to try to present to the American people both sides of a complex issue, and then to draw a conclusion. I had an abhorrence of presenting overly simplistic answers to complicated questions. That made it very difficult for me to communicate as effectively as President Reagan does. He does not immerse himself in the intricacies or the complexities of issues. He depends upon his subordinates to do that."

Carter is told that he hasn't answered the question. Who's the better politician -- himself or Ronald Reagan?

"It depends on how you measure a politician," he said. "I think his simplistic approach makes it possible for him to communicate very well. But in the long run, I doubt if it will be effective."

He is then asked to name the best living politician in America. He's stumped.

"Hmmmm," he said. "You mean at the presidential level? At the gubernatorial level?" Any level he wants, he is told.

He pauses for a long time. Finally: "I think a governor like Jim Hunt [of North Carolina] is highly popular in the state, and he's not avoided any controversial issues . . . my successor in Georgia, George Busbee, is a superb politician. He's not extremely innovative in nature, but he's consolidated the radical changes that I implemented. He's extremely popular . . . I'm not trying to be evasive, but it's a difficult question. Is the best politician the one whose only goal is to be reelected? Or is the best politician the one whose only goal is to utilize what power and authority you have for a term to deal most effectively with the responsibilities that you've been elected to address?"

Use the second definition, he's told.

Carter pauses for several seconds, but finally: "I think with that question, my administration will be judged well."

"The thing that's sad for us who knew him," says a former member of Carter's administration, "is that he's such a damn decent guy, so much of a better, nicer human being than he seems. This book was a chance for him to show some of that. But for whatever reason, Carter left himself vulnerable to be criticized by people who wanted to criticize." Some of those people have said he failed to be analytical, omitted embarrassments and glossed over scandals.

"It was difficult to go into those areas of my administration that were unpleasant to me," Carter said, "or where I didn't succeed in a major undertaking. I had a tendency at first to kind of skim over those and to concentrate on things that were successful and gratifying. I would be very skimpy, and my editors would say, 'This is not adequate, go back into this, why did you do this, what made you decide that?'

"I would say that primarily in the last few chapters -- when I was writing about the Soviet invasion of Afghanistan, the holding of the hostages, the elections, the debate with Reagan, the aftermath of the defeat -- just to kind of resurrect those feelings of pain and disappointment and alienation and rejection, during and after the election, was one of the most painful parts of the book."

Carter insists he's not bitter about his defeat, although he will say that his wife, Rosalynn, was. "But it wasn't anything that was serious," he said. "After the election, we went to Camp David and had a very pleasant time. We hadn't seen each other in a long time. And then we came back to the White House. I was immediately immersed in very demanding duties. We had a very productive lame-duck session . . . and I had to deal every day, almost every hour, with the hostage crisis. I was busy, and she wasn't. Because for a lame-duck first lady, you can't open up a new career of dealing with mentally retarded children, or the aged, or rebuilding communities. So it was a letdown for her.

"She was constrained to make more of the plans for our retired life than I was. I was still the president, and very much so. She had to assume the responsibilities of leaving the White House, of a wife . . . she was the one responsible for packing all our stuff, for deciding what went into the archives, and what went into our house. She had terrible duties."

The effect on their marriage? "Our marriage hasn't been threatened or damaged at all by the political defeat," he said. "If anything, it brought us closer together. And we've had a good life in Plains. We have a chance to be closer to Amy . . . she's arriving at the age, now she's 15, where she's drawn closer to us than she has been in the past. We've got our financial difficulties straightened out. We have a good and exciting career ahead of us."

Carter rejects the notion that the balance of the marriage has changed. "I don't think so," he said. "Obviously, when I was president, I had to make the ultimate decision, and Rosalynn had to play a much more secondary role . . . Now, when we're back home, we're much more equal in our partnership. Sure. But that's the way we've always lived. In the first few months of our married life, I was the dominant person. I had traveled around the world, had been to the Naval Academy, and so forth. Rosalynn had pretty much been a hometown girl, but then it was just a few months before we were a full, equal partnership -- sometimes against my wishes. Rosalynn's a strong-willed person."

Rosalynn Carter is now working on her own book, due out next fall. Carter, smiling slightly, remarked: "It will undoubtedly sell a lot more than mine will."

The Washington names in the index of "Keeping Faith" are few. Numerous former Carter advisers have been amazed at the short shrift he gave his staff. "That's Carter," some of them shrug, saying that he was never a president who bestowed excessive or frequent praise. One former adviser explained it this way: "To Carter, those of us who can take care of ourselves -- like you and me -- don't need him. He doesn't have to worry about you. He said it so many times -- you and I don't need government. You and I don't need help. The job is to take care of someone who needs it."

Others have a more bitter view, saying that Carter was a cold president, isolated from his staff by his own choice. "Carter isn't proud of bringing people into the world," a mid-level staff member not mentioned in the book complains. "He was never proud enough to say -- 'I groomed him, I made him.' It was like one of the burdens of the presidency is to have a staff."

Carter explained it this way:

"When I prepared to write the book, I did it in my careful, engineering, meticulous way. I met with probably 15 presidential biographers or scholars who have made a lifetime study of presidential memoirs and biographies . . . Their strong advice to me was: Don't try to name everyone in Washington. The fewer the people that you can list in the book, the better the average reader will appreciate the book . . . Some of the more famous biographers of presidents, back as far as Thomas Jefferson and Teddy Roosevelt, and as recently as Johnson and Truman, said . . . 'There may be some people disappointed when they look in the index and don't find their name, but don't clutter up the book just so you can mention everybody who worked with you in the White House.' "

Some members of his own party are also annoyed with Carter, particularly for a statement he made on the "Today" show last month. One week before the mid-term election, Carter said that "the Democratic National Committee quite often is more of a burden on a nominee than it is an asset to him." Some at the DNC headquarters here were demoralized.

Carter said yesterday that his comment was misinterpreted. What he actually meant, he said, was that the party conventions were a hindrance to a candidate, particularly "where the contentions of the Democratic conventions carried over to the general elections."

Why does he think the Democrats dislike him?

"I'm not sure they do," he said. "Some do, some don't . . . but I don't think this is contrary to what's always been the case in the Democratic Party. There've always been people who strongly disapproved of Lyndon Johnson, who strongly disapproved of Harry Truman, and so forth . . . I don't think it's unique to me at all."

On the subject of the press, he said he's mellowed some -- although he readily complained yesterday about coverage during his administration. "There's a lot of irresponsibility out there," he said. "I know the heritage of fame and fortune for investigative reporters that came out of Vietnam, that came out of Watergate. But where was the investigative reporter who tried to say, 'Did Jimmy Carter and Rosalynn Carter cheat and lie and violate the law by trying to wash money through their warehouse and finance a political campaign?' We didn't. But there was never a reporter who came to me and said, 'Mr. President, did you do it?' . . . The other side is, when I compare my own treatment by the press with the treatment that was given to Lyndon Johnson, or to Richard Nixon, or to Harry Truman, or to Jerry Ford, or further back, to Abraham Lincoln or Thomas Jefferson, I came out okay."

Carter also said he has "no hope, no ambition" to ever run for president again. "I've served my time," he said. "I've held the highest elective office in this land, perhaps in the world . . . Should we get Democrats in the White House, I'll be available on an ad hoc basis to maybe carry out some important assignments. If I don't, it won't be looked upon by me as a deprivation in my life."

Washington is Carter's fourth city on a six-city book tour; he has been to Atlanta, New York and Chicago, and still has to go to Boston and Los Angeles. The amount of his participation on such a tour, as a former president, is unprecedented.

He said it doesn't bother him. "I've never been on the Phil Donahue show, but I was on yesterday. And I kind of enjoyed that."

Or as he put it: "I'm just a plain old former president."

Carter finished his run around Hains Point at about 7:15, got back into the limousine and then wiped his brow with a towel. "I'll confess," he had said earlier, "that I've never had one constructive thought while I was running. I just can't."

On the way back, the car stopped at a light right in front of the White House on the Pennsylvania Avenue side. "It looks good," he said. "But Rosalynn and Amy and I were never on this side. There were always the tourists and all the demonstrators. We lived mostly on the south side."

The limousine pulled up in front of Jackson Place, and Carter went in, showered and headed over to have dinner with Rafshoon and his wife, Eden, at Nora's, the trendy Dupont Circle restaurant for journalists and political types. It's a place where everyone makes it their business to know who's there, but when Carter slipped in hardly anyone noticed. He had a quiet dinner at table 10 in the corner.

When he left, the 15 or so people waiting near the bar finally took note of him. "Great to see you," some of them said, Nora's owner recalled. Carter smiled. And left.