I was an ex cyclist in my mid 50s for 2 years after contracting (apologies...it must be getting a well worn story now ) TB, pneumonia, pleurisy then hives as I was so run down.
1st year back I managed around 200 miles total, 2nd maybe 1000 and so on.
Each year gets a bit better, at 60 now I'm determined es to keep cycling but it'll never be anything like it used to be (50 mile rides at 16 to 18 mph average, 80 mile ride maximum, push push push. ) nowadays it's more like 20 to 30 mile rides at a more modest 14 mph..,
I've come close mentally to packing it In a few times...but I get a bit stir crazy if I do t have anything to do.