Good for you for knowing what you want and asking for help. I hope others will chime in with their thoughts. In the meantime I will get the conversation started.
You have the benefit of age and wisdom and more circumspection and are wise to seek to make the learning curve as steep as possible. If you’ve read our book(s) or spent anytime on this site you already understand that that your fitness plays a big role in afely enjoying the mountains. So, I won’t beat that horse to death; only saying that Training for the New Alpinism should be on your reading list. From it you can build your own training plan. We also sell training plans for those who want to have the plan laid out for them. The one I would recommend is the 24 week Expeditionary plan as it most closely follows the book. Fitness is not all there is though. Even with modest mountaineering goals you will need to develop a set of skills that have no relation to any other sports activity you have participated in. There is only one way and one place to acquire those skills. That is by learning and practicing in mountain terrain. I would suggest acquiring a copy of Freedom of the Hills as it provides the broadest brush look at the full spectrum of mountaineering skills.
After familiarizing yourself with the concepts in these two books you will need to find friends with similar interests whom you can learn with (the slow route to competence), join a mountaineering club that offers skills classes (slightly faster route but fraught with its own problems), or hire a professional guide to teach you these skills (expensive but definitely the fastest way).
I hope this helps get the wheels turning. Welcome to the fold.