Well, I was 71 when I made the transition from Kick Around the Neighborhood to Cyclist. I too was fairly fit and did weights. So, I was a bit older but still in the same ballpark.

I quickly discovered the old adage that you aren't competent until you've done a task 10,000 times is true. So, I abandoned any attempt at the currently in vogue training methods. Instead I did two things. First, I rode a lot. Well, as much as the climate would let me which wasn't that much actually; about a 1000 miles that summer. Second, I resurrected my old running training methods that had been successful. I used Effort of Breathing as a measure of exercise effort. I made sure I included all breathing regimes in every ride. Third, I really, really, paid attention to my diet. That was easy because I was already following excellent rules.

The people who promote specific training regimens mostly confuse what is necessary for racers with what is useful for people who just want to be all they can be. Racers are spending a lot of time working at the margins. This isn't a competition with anyone else but ones self. Folks like me, and I assume you, are working at the mainstream. Important to have the proper mindset.

Good fortune to you.