I think I need to keep in mind my age too. Two weeks ago I wanted to use the backhoe and I was home alone and pinned up the thumb out of my way and that triggered a set back. It is so hard to limit myself when I get to feeling better and some much to get down before winter temps get here. Tomorrow I have taken off to get the gutters on the horse shed that and finish the related dirt work (filling the stall area some) and get the fencing back in place. The gates and things can be addressed later but the gutters are going to help keep down the water flow in the lot. We got the barn on the high spot and on a filled pad of high clay content just to insure the barn will always be dry and no water pulling under the tack/feed room.
There is still two or three positions that I can get in that the pain shuts me down but thankfully it is not bad in my normal daily routines and the tractors are not any problems.
It is not necessarily about age, but it is also about what changes have happened while you were less able and "laid up", as I have been finding out for my self.
While I have known (intellectually) for a long time limiting activities quickly causes atrophy, loss of exercise tolerance, weakness, and other debilitating effects, and theyconly slowly reverse themselves when activity is resumed.
I thought I was well over all the weakness and windedness I had been experiencing right after they removed my knee for the infection, but while I had improved, it wasn't anywhere near back to normal until I got my knee replaced again and started to be able to use it, even though forcthe first 6weeks, my weightbearing on it was limited to only 50%.
Now that my surgeon has given me the ok to increase slowly to full weightbearing and PT has ok'd me for stairs and more, I really am slowly getting both my wind and strength back, but as you have already said, it is all too often hard to gauge where to draw the line between enough and too much, and sometimes we pay for having too much confidence, (or as my wife says, too little sense).
Keep at it, albeit slowly enough to avoid unnecessary setbacks and the possibility of further injury, and you (and I) will get back to normal eventually, although it may be a "new normal" depending on what heals and what scarring is left.
Best wishes, and continied prayers,
Thomas