Defending champion Herb Lorenz of Willingboro, N.J., running under what he termed ideal conditions, easily won The Athletics Congress/Penn Mutual U.S. National 20-kilometer road running championship for "masters" (40 years old and above) yesterday at Hains Point.

Lorenz, a 41-year-old school teacher, finished first with a time of 1 hour 4 minutes 42 seconds, nearly three minutes ahead of second-place Bob Fischer of Newark, N.J. The tall, smooth-striding Lorenz, running virtually alone much of the race, broke his own national record for the 40-44 age group that he set last year on the same course.

Trudie Rapp of Alexandria, the defending woman's champ, also broke her own age-group record as she outdueled Karen Scannell of San Francisco. Rapp, 43, shaved more than two minutes off her old record in the 40-44 group in finishing with a time of 1:18.02, 21 seconds ahead of Scannell. Rapp placed 31st in the field of more than 300 runners, about 40 of them women.

"These are probably the best possible conditions for a race like this," said Lorenz in accepting the first-place trophy. The weather (sunny and cool) is great and this course is nice and flat. You can die and still look good."

Two other runners set age-group marks. Linda Sipprelle of Bethlehem, Pa., the third-place female finisher, established the record for the women's 45-49 group with a time of 1:20.10, cutting more than six minutes off the old mark. And 73-year-old Eddie Benham, a former jockey, delighted the crowd and his fellow contestants by smashing the record for the men's 70-74 age group. Benham, who lives in Ocean City, Md., broke the old mark by more than four minutes with his time of 1:32.45.

Unlike most runners in masters competition, who have been running for a substantial part of their lives, Benham is a newcomer to the sport.

"I retired in 1976, and then my wife died, and I had nothing to do," said the short, slight Benham as he accepted congratulations from other runners. "So about two years ago, I started running. I thought it would be fun, you know, so I got my boys together -- one's 50 and the other's 28 -- and we began running together. I always kept in pretty good shape anyway, and so it came pretty easy. I won a $50 certificate in the first race I won, and I said to myself, "This is for me.'"

Meet official Tony Diamond noted that the number of entries was triple that of last year's meet. "The whole masters program is only 10 years old anyway, and it's growing in leaps and bounds," he said. "A lot of people don't know about it. The masters program gives you a chance to commpete against people of your own age group. It's fairer and a lot more fun."