Retiring as a sports car racer at 36, after sixteen years of phenomenal success, Mark Donohue elegizes: ""It's all over now. Gone are the apprehensions, the immediate failures, the hard-earned successes, the victory celebrations, the fans, the admiring kids, the congratulations, the enthusiasm. . . . All race drivers return to the obscurity they came from, but the gigantic leap between is absolutely breathtaking."" Donohue's motivation to be a great performer seems to spring from his heart as simply as Fred Astaire's spirit springs from his toes: racing is what he does best. What sapped him, as much as age, was not his first bad crash, in mid-1972, but a feeling of being overextended in his responsibilities -- and of having peaked. Competing was no longer just a matter of getting into the best car (his ""unfair advantage"") and pushing his skill to the limit; it was having seven cars and a team of four drivers. The business end of racing now demands that he forget his trophies and settle for the hassle of meeting factory representatives, setting up schedules and so on. And it has left him with mental anguish: he knows he could get out on the track and test every auto for its special needs, but he must allow his less-experienced staff to find and develop their own nerve-connections to the cars. All told, Donohue's story is far less exciting than Bill Libby's recent A.J. Foyt, but it's an amateur's dream of glorious hardware.