In yesterday's editions, The Washington Post incorrectly reported the starting time of the Old Dominion riding club 100-mile endurance race Saturday. The correct time is 4 a.m.

At 4 p.m. Saturday, there begins in Leesburg an event that most of us regard as athletic lunacy. The mere mention tends to leave one limp, or scrambling for a hammock. Or does the idea of running roughly the distance from RFK Stadium to Memorial Stadium in Baltimore - and then back - sound appealing?

This will be the sixth year the Old Dominion riding club has sponsored a 100-mile endurance competition, with a 50-miler for horses and horsepersons either timid or in love with sprints. Now it has decided to allow humans to try and run the 100-mile trail.

"We want to encourage multipurpose trails," said Ann Lewis, an official who also will be riding in the 100-miler. "We think there ought to be trails that everyone - hikers, bikers, horses, joggers - can use. We wanted to show this by having riders and runners in our event."

Even passionate runners estimated no more than token participation. How many persons, after all, can be interested in something nearly four times as long as a marathon? Probably at least five; possibly as many as 25, if every ultramarathoner in North America decided to run.

"Forty-eight," Pete Fields was saying yesterday. "About a month ago, we had 25. Then we figured we could handle 36. Then when we had that, plus a waiting list of six, we took a long, deep breath and upped it again.

"The medical protection is the major concern, what limits the number of runners. We just don't have the medical manpower for more than 50. I think that's it."

The entries were expanded after a teen-ager, both shocked and miffed at being told his name would be placed on the waiting list, said: "What problem? I just like to run. I don't want to get high. I don't want to get drunk. I just want to run."

"That touched me," said Fields.

One of the ironies, or at least to a jogger scarcely able to go 40 minutes before his legs wilt, is that the horses are required to be rested a total of four hours during the 100-mile event but the humans are not.

For that reason, the fastest runner might well beat the fastest horse.

"Runners are not stupid," said Ed Ayres, editor of Running Times who has helped advise the horse-oriented officials as they planned the additional competition. "It's been my experience, in ultramarathons and observing them, that the runner loses his motivation before he loses his judgment.

"These are adventures, like an expedition or a safari, and this one ought to be especially enjoyable because the trail (through Loudoun County and part of Clarke County) goes through such beautiful countryside.

"This is a tough course, but sometimes the worry about runners becoming disoriented is too great. What was it about the world ending with a whimper? Well, it's sort of the same here. A person simply stops running. There he is, along the side of the road, limp but physically okay."

At Morven Park in Leesburg the runners will be starting an hour before the riders - and not because the reverse would produce an extraordinary amount of bobbing around road apples.

"The runners aren't so concerned about being overtaken by the riders," Fields said, "because they're sure they'll be able to hear the horses coming. But if they were to overtake horses, they'd be doing it more quietly - and possibly would spook them.

"The second reason is that even in the dark they (the runners) wanted to know there were both horses and people in front and in back of them. Four runners have said they plan to be finished in 15 hours or so."

Which, given the mandatory stop, is about when Dr. Matthew McKay-Smith ought to be riding to victory. If he should fail to win, it would be a major upset, for he holds the course record and works terribly hard at these events.

He has a pit crew.

A what?

"People with trucks, or vans, that carry hay and grain and meet the riders at their stops and take over, feeding you and also the horse," Lewis said. "Sometimes they have special trucks with hoses, so they can hose a horse down along the trail. Those people are going for speed."

Lewis and many of the other 45 to 50 riders entered in the 100-mile event (there are an equal number in the 50-miler) are in the cavalry competition. The incentive there is to complete the 100 miles in 24 hours, along with absolutely no outside help.

"The reward is just getting through in one piece," she said, adding that a silver belt buckle also is given to the winner "Can you imagine going through all the heat - and last year it was raining - and finishing the next morning and then not receiving a buckle because the horse failed to pass the final vet check?

"It's crushing."

This will be Lewis's second year in competition, because her horse went lame during training the first time she entered. Last year, the horse cramped at 50 miles, "because I'd gone out too fast," she said.

Saturday, she plans to start in the last group (they go off in groups of 10 at about two-minute intervals), carrying as little as possible. She hopes to finish by dark. She will take a flashlight along.