It may help to adjust cadence or fit. You aren't giving your muscles time to recover from one hard effort before doing another, and that doesn't make you stronger, it just breaks you down. If you want to get better, ease off a little. Massage those legs a bit after riding. You can also try massaging your legs with a roller.

The older we get, the longer it takes to recover. Just don't do all the rides at a level that pushes you above what you can manage. Its better to ride with slower riders when you want to put distances in. Good advice above but I'd like to emphasize taking a recovery ride the day after you do a hard ride.