The first thing I'd do is to get a variety of medical opinions about any issues I have and how exercise can be best used for me. In my case, I am 76 and have reduced the length of my rides from 30-50 miles to 15-30 miles. I've also increased the pace/intensity. That is not to say that my average MPH is up but just that I pick my spots to see how well I'm riding compared to years gone by. I am an ex-college athlete and the attitude of trying to have accomplishments and improvements suits my psyche. My typical ride might be something like 15-20 miles with 1,000-1,300 ft of climbing. On a ride like that I'm usually around 15 mph.