Knackered - How old is too old?

No I'm not exactly knackered at the moment but how on earth can I keep on going for much longer? Actually there was a point at which it was doubtful that I would be able to continue window cleaning for much longer. I would have liked to have retired sooner but things are as they are. So how have I managed to keep going and stay relatively fit? In this video I give a brief explanation of just how I did it.