I might just buy a new diesel van in 2029 and run it until retirement...I'll be 59 by then with probably 20 years left(I'll retire fully by 80 health permitting of course)
The best laid plans of mice and men......
The guy I used to work for back in 1992 is still going and so is the other local cleaner(79 and 78 respectively).they look great for their age and are fit and healthy.they both only work mornings though these days... 
They are outliers Daz. The average person is nowhere near capable of doing a manual job at 75+ years of age. It’s possible, but I wouldn’t set your retirement plans on it happening.
I don't class myself as an 'average' person as you put it.....most of my drinking buddies and a few school mates are already dead mate unfortunately.....not everyone makes it to 50 years of age....
Working outside and doing an active job keeps us healthy.....its sitting down all day that knocks the nails in your coffin early!
I’d bet good money that you’ve had more time off work with muscular injuries in the last 5 years that the average 50 year old manual worker. Tweeking your back, foot injuries etc etc
You may think you’re invincible Daz but all these niggles you’ve had will come back to bite you in the ass when you get to 70+.
Good luck anyway, personally I hope I’m long retired and comfortable when I’m 75.
I've had 2 weeks off in 5 years with slight injuries....one with my back and one with my foot....
Granted there are no guarantees but your making out as if window cleaning is a heavy manual job which it isnt....its easy these days with light poles and no collecting....
and the beauty of it is you can cut down your hours to suit...
I never want to retire fully and I won't until I'm forced to through illness or injury.....
My dad was still working at 74 before lung cancer killed him(63 years of smoking and he drank fairly heavily)he d never been ill in his life before then!