At the tender age of 53 I can say that fitness is not a major motivation for cycling; I'm never going to be a top athlete! It is a nice by-product however. My main reason for cycling is because it is a pleasant stress free way of getting around, and you get to see and appreciate so much more when cycling rather than driving. Factor in wind and rain, and for me, cycling is just another stressful unpleasant experience - so I don't do it!
With you on the last sentence although my only real motivation
is fitness.
I do enjoy cycling to an extent but jesus it's pretty boring and repetitive.
I just don't have the right mindset to crunch out thousands and thousands of miles a year. I admire what people do as it takes a lot of dedication and willpower but it's not for me. We have a guy locally that is on his bike 4 or 5 hours of the day every day no matter what. He's widowed, no local family and few friends so it works for him and he loves it so I can't knock it.
I'm not sure where, or even if there is one, the line between fitness and wearing out your body lies.