Dan O'Connor of Westminster, Calif., mastering what he later characterized as "a death march," took the lead after 15 of the 20 laps and won the TAC National 50-kilometer race-walking championship yesterday on a course around Constitution Gardens.

O'Connor's time was 4 hours 22 minutes 22 seconds. This was his 10th start in this championship race and his first victory after three seconds and two thirds.

O'Connor, 30, a member of the 1980 Olympic team, passed leader Ray Sharp of Colorado Springs after 37 1/2 kilometers. Sharp had set a fast pace early in the race but was forced to retire due to dehydration after 17 circuits of the 2.5-kilometer route.

Once ahead, O'Connor pulled away from Vincent O'Sullivan of Flushing, N.Y., the runner-up in 4:25.02.

The top performance from an area athlete came from Sal Corralo, 51, of Potomac Valley Track Club. Corralo's time of 5:07.01 broke the record for his age by six seconds. He is also the record holder for ages 47, 48, 49 and 50. He placed 16th overall.

The course was manned by six judges, each accompanied by a volunteer shortwave radio operator. The judges issued first cautions, then warnings, and finally disqualifications to several competitors caught creeping or lifting. In race walking the forward leg must be fully extended and the knee locked with each step in order to avoid creeping. The racer must keep one foot on the ground at all times or else he will be accused of lifting.

The top nine finishers met the qualifying standard of 4:50 in yesterday's race to join the team representing the United States in European competition this summer.