Whenever one's energy wanes, it is good, for purely inspirational reasons, to visit 65-year-old Harry Walker, a king of the lecture business, and his son Don, the apparent heir.

Name a hotshot on the lecture circuit and Harry's got him: Gerald Ford, Jimmy Carter, Henry Kissinger, Barbara Walters, Alan Greenspan, and "practically the whole Carter Cabinet." Whom he cannot have, for reasons of this irritating government regulation or that, you can bet he one day is going to get. Here are two reasons why: Harry Walker, in addition to 10 toes and 10 fingers, was born with a phone in either hand ("It's an operator!" the doctor probably said), and Harry Walker does not sit around and wait for those phones to ring. He's hanging out with Menachem Begin in the lobby of the King David Hotel in Jerusalem, and though Begin will not be able to lecture for money until he leaves office, Harry Walker--an old friend, as it happens--plants the idea.

"Menachem," Harry says, "you left government right now, what would you be worth?"

"Maybe $1,000," says Begin.

"You come to me," Harry says, "I can get you $15,000 a night."

He relates this, Walker the younger at his side, at his offices on the 36th floor of the Empire State Building. Some with a prestigious clientele might go for a little flash, but the offices of Walker, Inc., despite the autographed photos and the panoramic view, are scruffy, crowded and homey. The tour, by Don, includes introductions to the entire staff, including the bookkeepers and the mailroom clerks.

There is, as well, an apparent family connection. Harry, in his bold pin-striped suits, has thick-framed black glasses and a mighty schnoz. Don, at 30, is in some ways the shadow of his father, drawn with a more delicate brush. His nose is the acorn to his father's gnarled oak, but with the same promise. His black-framed glasses are a slimmer version of his dad's.

They complement each other, they will both say separately, because they are opposites. Harry is the more garrulous, wheeling and dealing on the phone, bragging about the "five tremendous dates" he booked for Jimmy Carter, where, naturally, "he made a tremendous hit." Don is quieter, with the patience to listen to the people who call every minute, or to remember, say, that Louis Rukeyser always has to have a king-sized bed with a board.

"My youngest," says Harry, fondly, "also, the one who eats less than any of them. . . "

They do well for their people. When candidate Reagan declared his income, such were the revenues he declared from Walker, Inc., that The Wall Street Journal called to inquire. Likewise, Alexander Haig, between leaving his post as head of NATO and going into the private sector, made half a million bucks.

"Then he had a triple bypass," says Don the Younger, "but it was after he went to United Technology so we blame it on the rigorous corporate life."

Haig's photo, with inscription, still hangs on the wall, as do the photos of other politicians, including former vice president Walter Mondale, posing with Harry arm in arm, before losing the election.

"I remember he said, 'Harry, I might need you, I better hold on,' " Harry recalls. "It's a funny thing, I stood next to George Bush, he said the same thing."

"How do you know who's hot?" people always ask Harry. "How did you get who you got?" The answer, according to Walker elder and younger, is no big mystery. They know who's hot from watching the headlines and news shows (Harry has three Betamaxes at home on Long Island, and two in Miami Beach).

They get who they get because, frankly speaking, Harry has been in business 35 years, starting back in Boston booking college professors for garden clubs, and by this time he knows almost everybody. He sees Robert White, former ambassador to El Salvador, on the "Today" show, and he can book him over the phone before he leaves the studio--Tom Brokaw is a client, so Walker can get right through. Richard Allen, the minute he left office, Harry has him on the phone. This is not, either, because he needs the money. It is, as Don points out, for fun.

"He just loves the business, my father," says Walker the younger.

"Like what do the mountain climbers say about Everest?" says Walker the elder. "How they do it because it's there."

He has, Harry volunteers, made mistakes. Years ago Moshe Dayan called to see him--he was just a general then--and Harry told him to come to the office, kept him waiting, and then said he couldn't get him much, maybe a few college dates. A year or two there's the '67 war. Harry wires Moshe immediately: "As La Guardia said, 'When I make a mistake, it's a beaut.' " When Dayan leaves office, Harry signs him up.

Changes in administration, Harry says, are always good for business--even a little coup. He remembers the day a few years ago he took Don and went to Washington and signed up Mondale, Secretary of State Edmund S. Muskie, National Security Adviser Zbigniew Brzezinski, Treasury Secretary William Miller, Carter confidant Hamilton Jordan and independent presidential candidate John Anderson--the most important men in Washington, all in one day.

Now, he's got other fish to fry. Jeane Kirkpatrick at the U.N., he'd like to make a little contact there, then when the time is right, he'll have a client; maybe Sen. Pat Moynihan will intercede. He's still working on Begin. He made the first mention of the lecture business to Menachem four years ago. "$15,000 a night? I could leave a rich widow," Begin had said then, in reply. Now every time Begin runs into Harry, he jokes. But he remembers. Harry remembers, too. He watches the papers. He sees the news nightly. He looks for a change. Perhaps a change of government. Begin is frequently in his thoughts.

"The government topples," Walker says, "he's got a job."