Play on middle tennis courts
OK. You know you need more time between points than your in shape opponent. You’re playing outdoors.
Choose the middle courts whenever possible. This automatically builds in lots of extra time as you retrieve balls that roll to the side of the court because there is no fence to stop them.
This works even better when no one else is playing on courts next to you because the ball could roll a couple courts down and no rule anywhere says you have to run to retrieve it.
An added benefit that admittedly is a little “gamesmenship” is that your in shape opponent is likely to get a little irritated with these “built in” delays. Irritated tennis players make mistakes. This is good for you.
Also, when they have to chase the ball on their side of the court, they most likely will run to retrieve it so that they make a point to you to hurry up. This just makes them breathe heavier and opponents breathing heavy make mistakes. This is good for you too.