Make ’em hit overheads
It’s really tiring for the average overweight and out of shape club player to hit overheads. They have to hurry to the ball. They have to keep their feet moving to get into position and they have to make a large motion to hit the ball. Most players also have to use their opposite hand to point to the ball to avoid a mishit. All of this motion burns lots of energy.
Even if you hit some short lobs and get killed with the overhead smash, you have forced your opponent to do all the above to burn energy, plus the short lob made them run extra far to get to the ball.
Lobbing to the opponents backhand makes it even more difficult to get into position which causes them to burn even more energy.
Lob a lot early in the match on out of shape opponents and watch them fall apart quickly.