Jim Alexander, 24, the proprietor of a running footwear store in suburban Philadelphia, is expected to challenge former winners Will Albers and Robert D. Hirst in the third annual D.C. Marathon Sunday.

Alexander's best marathon was 2 hours 20 minutes in Montreal in 1981, almost eight minutes faster than the course record of 2:27.58 for the D.C. Marathon set by Albers the first time the race was held.

But Alexander's time is still more than two minutes off Albers' best marathon time of 2:17.50.

As of yesterday, 965 runners had signed up for the race, compared to last year's 725, according to race director Samuel LaBeach.

By Sunday, LaBeach said, he expects more than 1,000 runners to have registered for the 26-mile, 385-yard race that follows a hilly and circuitous course through all eight of the city's wards.

Alexander, who has run in the Boston Marathon, said he elected to forego that race Monday to compete in the marathon here, "because I was looking for a major, but lower-key, marathon.

"I thought this would be a nice time to get a tour of the city, but at the pace I'll be going I doubt if I'll see much. I'm going down there basically to win, so hopefully I'll perform.

"I really would like to qualify for the Olympic trials, which is 2:19.04, and I thought that by running a lower-key race I could build myself up.

"I'm set for a pretty good race, and I think I'll be ready. I ran in a blinding snowstorm last night for about 17 miles."

Both Albers and Hirst, who battled Albers for 26 miles before pulling ahead of him to win last year's race, have registered to compete again this year, as has Patricia Howard, last year's women winner.

This year's race, which begins at 8 a.m. on the day that clocks are turned ahead an hour for daylight-saving time, will include seven wheelchair contestants from the Capital Wheelchair Athletic Club, race walkers and teams from five Washington area running clubs--the Washington Running Club, the Potomac Valley Seniors, the American University Eagles, the Rock Creek Running Club and the Naval Research Lab.

Ed Benham, 75, of Ocean City, a veteran of the city's two previous marathons as well the autumn Marine Corps Marathon, is due back in an effort to improve his 1982 time of 3:35.56.

For the first-place male and female winners the prize will be a color television set. Additionally, said LaBeach, the oldest finisher will get a television set, and one television set will be awarded at random to a race participant.

This is the latest the D.C. Marathon has been held, and LaBeach said the contest next year probably will be changed to a late March date, as it was a year ago.

"We just didn't want to conflict with Easter or the Cherry Blossom race, but whenever you do it, you're going to be running into problems," said LaBeach.

"If this cold weather continues, it should be ideal Sunday. Anything from 40 to 65 degrees is good. Anything over 65 degrees is not too good."

Beginning on the Mall, the runners will follow a course that will take them through Georgetown, Cleveland Park and Adams Morgan to Northeast Washington, Catholic University and Brookland.

They will cross the Anacostia River on the Benning Road Bridge, return on the 11th Street Bridge and follow Pennsylvania Avenue past the Capitol to the finish at the Museum of Natural History.