Don't know how old you are, but in my 60s I can promise from experience that things will only get worse. Twenty years ago, I did my first century on zero training, just an occasional 10-15 mile ride. A friend phoned on a Friday night and asked me to ride with him on Saturday, and I made it. I probably won't do any more, but if I do, I'll have to train for months.
Possibly related note: I had a long hospital stay last fall with four weeks of almost complete bed rest. The docs told me that each day in bed requires three or four days of rehab to regain full strength. That's been accurate in my case--I started therapy in mid-November, and I'm just now getting most of my function back.