As others have said: Lift Weights!!! I'll add: Do it often and do it heavy. Get instruction on how to do it so you don't hurt yourself.
Right after I turned 60, some 10 years ago, I started a new "career" in EMS and Fire. One of the guys turned me on to freeweights. He showed me how and encouraged me to read everything on the subject. The result is that I have no trouble at all with muscle mass or strength. In fact I've had to tone down my workouts to something about 1/3 less than what I was doing so I don't have any excess muscle to move on my bike.
One doesn't have to want to look like those mountains in Muscle & Fitness to use their techniques to good advantage. After all we want to ride bikes and every extra pound on the body is a drag.
Plus, it does the ego good to be able to work out with the "kids".