BATON ROUGE, La. (AP) _ Half of the National Sports Festival marathon might fall victim to the heat and humidity here, according to race director Len Bahr.

Bahr ran the course Sunday morning and called it ''brutal.'' He also said The Athletics Congress, the governing body for track and field in the United States, will evaluate the weather next Saturday night before deciding whether to hold a full marathon Sunday morning or cut it in half.

''I ran one loop of the course at 7 (a.m), which is about the time the second loop of the full marathon would begin,'' Bahr said. ''It was brutal.

''I was running at a very modest pace of about seven minutes a mile compared to a competitive time in a race which would be about five-something. It took me about an hour and 35 minutes. It was brutal and that was only one loop. So I'm quite concerned. There have been several people who have said the temperature has been overated as a factor and I say those people have never run a marathon, especially in July in Baton Rouge.''

Other lengthy events which have been scheduled for early morning to combat the temperatures include the men's 20 and 50-kilometer walks and several cycling competitions.