I regularly do 30 houses a day, I like working at a brisk pace, I’m not running around like a headless chicken though.
I’m similar in that if I only did 10-15 houses I’d have to double my prices, and only work mornings. Sounds pretty good mind you.
I’m 34 and physically feel great at work, I’d recommend seeing a physio and they will no doubt give you some light resistance training to build up weak muscles that are causing an imbalance.
Although people seem to say window cleaning is tough on the body and if you work at that pace for too long you will fall apart, I don’t agree.
Use the best equipment out there, and look after yourself.