When you're younger there's a tendency to overdo things and think that more is better. The logic is you think if someone else is riding 3 hours and you do 5 hours, progress will be faster. However, when you get older the aches and pains start to set in.
Based on my own experiences, I would suggest enthusiasm has to be tempered down and days off are essential. Joints need to be rested and the body needs time to recuperate. Above all, the temptation to go hell for leather on every ride needs to be modified.
Now I turned 40 I make an effort to resist some of the crazy excesses I used to go in for. Today, for example, I'll go out on a moderate ride, try and avoid too many hills and just get some basic miles in. Sometimes, though, I'll go really hard and push myself flat out. But if I feel I need it, I may take 2 whole days off and wind down.
I'll be honest, though, now I'm 40 my joints ache a lot because I used to train hard every time in the past.