Potomac Park will be overrun today by competitors in two amateur foot races.

At 9:30 a.m., about 400 women will break from the starting line in the second annual Eastern Regional Women's 10-Kilometer Championship in East Potomac Park. The action will shift to West Potomac Park at 2 p.m. when about 100 entrants are expected for the National AAU Senior and Masters Three-Kilometer team championship.

"If Julie Shea does show up, she will have to be the favorite to defend her title in the morning race," Jeff Darman, president of the Road-Runners Club of America, said last week.

Shea captured this event last October and set a record for American women by covering the 10 kilometers (6.2 miles) in 34 minutes 51 seconds.

"It probably will be a closer race if Julie doesn't come," Darman commented. "There are several women who would have a good shot at winning with her out of the way."

Shea, an 18-year-old freshman at the University of North Carolina, was entered in an intercollegiate race yesterday, raising questions about her participation in this morning's race.

Darman named Aileen O'Connor, 16, of College Park; Charlotte Lettis, 26, of Amherst, Mass; Carol Fridley, 29, of Harrisburg, Pa., and Julie's sister, Mary, 16, of Raleigh, N.C., as contenders for top honors. He also rated Jennifer White, 23, of ALexandria as an outstanding local entrant.

All entrants were required to register for this race by Sept. 20.

In the afternoon race, the New York Athletic Club is the clear favorite to gain top honors in the senior division of this 1.9-miles AAU event. The senior division consits of runners between the ages of 20 and 39.

"If NYAC shows up, they'll certainly run away with it," race director Mike Bradley predicted last week. "But they may go to Ontario this weekend to run in the Springbank International 12-mile race instead.

"If NYAC doesn't come, it will be between the Washington Running Club andthe Chaclottesville Track Club."

Top performers for NYAC include Matt Centrowitz of Manhattan College, who has run a 4:01 mile, and Mike Keough, formerly of Manhattan, who had a good shot at representing the U.S. in the 1976 Olympic 5,000 meters before he was injured prior to the Olympic trials.

Al Naylor, who broke 14 minutes in the three-mile race when he ran for Maryland, and Will Albers, formerly of George Mason, who has broken 50 minutes in the 10-mile run, will compete for the Washington Running Club.

"Without a doubt, the Potomac Valley Seniors will be the top team in the masters' division." Bradley remarked. This division includes runners who are 40 or older.

Anyone may enter this event by paying the $2 entry fee before race time, but team prizes will be awarded to the AAU teams whose top five finishes have the lowest cumulative times.