I agree with Terry. In addition: Interval training has been recommended to me with good results. I use a hydraulic trainer such as CycleOps Fl 2. On it I started at 15 MPH, 80 to 90 cadence for 15 minutes, get off the saddle and shift up 2 or 3 gears and sprint for 1 to 3 minutes at 17 to 18 MPH, repeat that for at least one hour. (these days I average 18 MPH)
I started doing that at age 60 so there is no reason not to do that.
You can do the same on a road but there are distractions. I train on the road at 17 MPH average and sprint at 20. Assuming flats. (these days I average 20 MPH on a flat road)