Sounds like a simple recovery thing. As in: not quite enough for the effort you're giving.
Same as with running, the "hard" days (ie, intervals, hills) require a bit more recovery time and suitable nutrition during the recovery period to effectively rebuilt what's been hammered. Plus, at nearly 60yrs of age, you're not a "spring chicken" anymore.
Might alter your cycling regimen a bit. Something like what many do with running: longer/slower rides on many days, with tougher/harder/climbing type rides interspersed, with a couple of days of recovery (doing longer/slower stuff) in between the tougher sessions. Weaving in other types of exercise can help, as well. Weight training or other weight-bearing exercising, occasional running, etc.
Be sure to have sufficient nutrients (including protein) following those hard sessions, to ensure you're giving the body what it needs to do the repairs.
Guesses, but not a bad place to start if you're definitely hitting most sessions harder.