Your own fitness is down to your lifestyle. I used to be one of those "Ultra" fit buggers up till I was 34. Then I had to give up running due to knee problems and a family. Took 10 years off sport but realised that I had to get back to doing something to retain the fitness I still had. Took up cycling and that was 20 years ago. The knee problem was still there but not when cycling. Took a couple of years before cycling fitness came in fully but then it was long distance rides that got my interest. Metrics and 100 milers----But offroad. If you want to find out where your limits are- get a mountain bike and do a 100 miler with plenty of hills.

But you can only push yourself that hard if you are fit-and I do mean fit. You have to keep training and training hard. Some of us cannot put that commitment in all the time. Work and family have a bearing on how much "Spare" time you have to spend on your own leisure time. I cannot put the time in to stay fully fit all the time but I have a 100 miler coming up in October. With my current fitness it will be a struggle but A few extra distance rides- a few extra hills and I will be there. If not then I will have leisurely ride within my capabilities and I will enjoy it.

This is the way I look at it. If you don't enjoy it- why Do it? I think that this philosophy works for most of us as we mature. If you just want to take rides in the country side at a leisurely pace- then fine. If you want to go out and blast for 50 miles- then fine again. But whatever you do- Enjoy it.