I have taken up biking (indoor and outdoors) and have entered many mountain bike races (29 mile ICEMAN) in northern Michigan. I have a routine where I am getting 10 miles in every other day staying under my target time of 40 minutes (under 4 minuted per mile). On days I do not ride, it is light weight training for flexibility.
My left knee needs replacement at some point (I am 57 YO). However, I feel the burn and feel great doing it. If you are resting, you're rusting, just like your car.
I still eat like sh!t and drink, but I can burn it off now.