I'm 72 and live at the top of Crouch Hill - still ride fixed (as well as geared), say 70/72ish gi depending on which bike. I don't see why riding fixed should be any more tiring than geared (in general, that is). You just need to be determined and, as you say, work harder.
Hi Bruno, you'll get more response if you put this in Classified, Components and Clothing with that pic you put on Instagram to give an idea of what you got. All the best, Alan
Issues with access for Spudger so continuing with Spudgertoo alternative