In my 50's I would average around 22 - 23mph. At 73 I can no longer do this unless in a group. The major thing I have found other than trying to ride 200 mi weeks with a lot of climbing with intervals thrown in is to go to the gym and use weight machines, especially quad and hamstring machines. Do quite a few reps as opposed to few at max output. Power will increase along with power endurance. We are all different but I do 40 reps at just under 400 lbs. on quads and hamstrings at around bicycling range of motion settings. I still have a few Strava firsts in 65+ but admittedly they disappear as I age and have to take meds which lower my max heart rate.