I find weight to be the biggest issue with PF. Not even fat - just plain weight. After getting under 200lbs my PF is just fading away.
#1. Don't play on consecutive days.
#2. Get yourself somewhat fit off-court.
#3. Gradually increase your play time..
Lots of people think playing tennis will get them fit. I think get yourself fit to play tennis. I suppose the second option is less fun. But that's how life is. Tennis is an impact sport..