The frail old man in a gray sweatshirt, blue work pants, white socks and steel-toed shoes walked jauntily across the prison yard, pausing to tap a fellow inmate on the shoulder or to look up when another greeted him as "Governor."

After nearly five months as an inmate at the federal prison camp here, Marvin Mandel has lost 15 pounds, acquired a Florida tan and come to learn, after a lifetime in the spotlight, that getting special treatment can work two ways.

"This is a prison, make no mistake about it," said Mandel as he escorted a visitor around the wall-less prison. Inside dormitory No. 4, Mandel walked to cubicle 9B, where he showed off a stack of 18 letters that he had received yesterday, a typical mail call for the still popular two-time chief executive of Maryland.

Sitting in his cube, as the inmates call the six-by-eight-foot areas, Mandel, at 60, puffing on his ever-present Meerschaum, looked like the lead in a latter-day George Raft prison movie.

Not the tough guy, but the elder statesman, the graying guy the other prisoners turn to for advice, which is not too far from the truth, as explained by Mandel during a rambling two-hour interview here today.

Mandel was happy to talk about any subject -- from politics to the plight of prisoners' families -- but the reason he had his wife Jeanne invite the press to interview him was because of "the special treatment I am getting here."

Mandel said that when he entered prison on May 19, "I was determined to keep a very low profile, just do what I had to, be one of the people, mind my own business," he said softly.

"I wanted to be treated just like anyone else, but apparently that won't be," said Mandel. After the federal parole board in effect denied him parole by saying Mandel should serve two years of his sentence for political corruption it provoked the governor to rekindle his old love-hate relationship with the press.

"I just got in writing yesterday" what he had been told in person a month ago by a parole examiner, that his release date will be May, 1982, Mandel said.

"It says because I was governor of the state of Maryland and because of the widespread publicity" given to his imprisonment after two trials and two years of appeals.

"I didn't understand the parole system," said Mandel feigning understanding, a hallmark of his long fight to avoid conviction.

"I thought the purpose of the parole system was to look at a man after he came into the institution, look at his action (crime), determine whether he is a danger to the community and whether he is likely to repeat his crime.

What he has learned about the federal parole system, Mandel said, that "in spite of what the judge might have done, who heard all the details, you go through another trial, and if the parole board decides the judge didn't give you enough time, they make you serve the full sentence."

When he came to Eglin, Mandel figured he would get out in a year. He was eligible for immediate parole under the terms of the sentence handed down by U.S. District Court Judge Robert L. Taylor.

Mandel said he was "not so naive" as to think he would be released quickly and expected, as an example, to get more than as a slap in the wrist. "I knew I was a hot potato," he said.

But at the age of 60, and with no release in sight, it's difficult. "It's not like tacking on another year to a kid of 22 or 23. At my age, a year means everything," he said.

"It's hard enough on me," he said, "but when Jeanne comes down for a weekend (which she does about once a month) it takes her a full week to recuperate."

The governor has had one opportunity to spend some time privately with his wife. He got a 15-hour pass (8 a.m. to 11 p.m.) on Sept. 21 and spent it with her in nearby Fort Walton Beach.

And while Mandel acts as an informal adviser to many of his fellow prisoners -- he teaches a Toastmaster's class on how to make a speech, was elected president of the Eglin Fellowship Group (keeping intact his record of having never lost an election, including 22 for local and state offices), attending Jewish services on Friday night and studying Spanish -- the solitude of prison life was apparent today. He appeared genuinely happy to talk to a reporter who had covered his criminal trials and dogged his steps for several years in the late 1970s.

"It's a prison," he repeated. "They have you on a string like a puppet and when they pull it, you move."

He ridiculed descriptions of Eglin as a country club for white-collar criminals, with its tennis and racquet ball courts and home box office movies on television.

"Some country club," he scoffed, nothing that his daily schedule begins with breakfast at 5:15 a.m.

"About 6, you clean up your cube, sweep the floor, make the bed," he went on. "Then you go to work. Everybody works from 8 to 11 and after lunch, again from 12 to 4. And then it's hurry to dinner, which ends at 4:45. Next it's back to the cube for a count (when prisoners stand by their beds as guards check off their names)."

The evening begins with mail call and, Mandel bragged, "I answer every letter." The next three hours are free time, during which Mandel reads (40 to 45 books so far) or writes letters until another "count" before lights out at 10:30.

Nearly every day Mandel sees Harry W. Rodgers, who works in the kitchen, but "thank God he doesn't cook. The food's bad enough," Mandel laughed.

Rodgers was one of four other men convicted of giving Mandel gifts worth more than $300,000 in return for which Mandel used his position as governor to enrich them through a racetrack they secretly owned.

Another codefendant sentenced to Eglin, Irv Kovens, has been transferred to Lexington, Ky., where, Mandel said, Kovens successfully underwent heart bypass surgery on Monday.

Mandel's own job for which he earned $13 last month (which he can spend only in the commissary where he buys his pipe tobacco) is in the laundry. He stencils a number on each set of clothing for new prisoners (his own number is 13) and then distributes them - four shirts, four sets of underwear, four trousers, three pairs of socks, one pair of steeltoed shoes and, for the yet-to-arrive fall weather, the gray sweatshirts.

"Next week, we pass out the thermal underwear," the governor said.

And then the interview was over. The prisoner left the conference room and in the excitement, turned the wrong way in the hall.

"Mandel," commanded a guard, "the other way." The prisoner shrugged, waved with his pipe and trudged back to the laundry.