A Silver Spring vegetarian who owns a computer company and dabbles in poetry, music and writing, L.D. Ridgeway decided two weeks ago to enter the Marine Corps marathon, his first.

"That's just the way it turned out. I was building up my distance at the same time the Marine Corps came up. I only decided to try and enter only two or three days ago," said Ridgeway last week. "I know you have to qual- ify for Boston and this is a local race. I'm not even aware of any other marathons."

A year and a half ago, Ridgeway's physician told him it was absolutely imperative he lose weight, no excuses accepted. He is 6 feet and at the time weighed about 220.

He altered his eating habits, took up aerobics and began to jog. Now he's down to 180.

"My distance just started to increase," he said. "I'm not a competitive-type person. I never thought in terms of running a marathon. I'd never even dreamed of it. I don't like racing. As a rule, I don't enjoy competitive-type situations. But, when I'd run three miles, I just didn't feel used up, so I'd run farther and just kept going up. Then, about two months ago, I ran 20 miles. And I thought, 'Geez, if I can run 20, I can run a marathon.' "

He travels extensively for his company and between raising two young children tries to get in about seven miles a day.

Now exercise is a way of life. As well as aerobics and running, Ridgeway lifts weights a few times a week.

"Physically, I feel terrible when I don't do anything, but it's visceral, emotional, also," he said. "And I know other people who are that way, too."