Thomas Muster came back to play Challengers after getting terribly out of shape and actually won a couple of matches and many of the matches were very competitive. Of course, that is at a level far higher than any woman, including Serena, plays. She probably wouldn't get more than a game or two if the Challenger guy relaxed a lot. Similarly, she would barely get a game from guys like Sampras or Rafter on the old guys tour. McEnroe is 8 years older than Muster and still plays a very high level of tennis, but even watching the tour on TV, I can tell he isn't as good as a few years ago. His game can still be mystifying, even to the former greats though, if he is playing well.
All in all, I'd say he'd still beat any woman pretty easily.