@enkidu: If at all possible I would highly recommend a real live TI hands-on coach. The books and videos are great, but a real live coach should be able to correct you in errors sooner than you might be able to figure out on your own, and you'll get to a level of expertise faster than without help.
I too am vary fit and strong for my age. I'm an experienced runner too, so it was a real shock for me to find out that this does not necessarily transfer to an easy acquisition of swimming skill. My problem is that I tend to put a lot of strength and force into my movements, which is not necessarily a good thing, in fact usually not, actually, in swimming, especially economical swimming. Fortunately my TI instructor that I see about once a week is able to take the edge off this tendency, and when it works, I'm better able to put my awareness to better use in the subtleties of TI stroke finesse, but it's a work in progress.
Interestingly I am learning Aikido, too, in my old age, and it's amazing how parallel a progression to TI swimming it is, complete with me forever using way too much force (from years of training in hard martial arts), and ruining the effect of of the technique I am trying to learn!! Again, having a real live instructor and real live experienced partners really helps to correct one's faults in real time, rather than weeks or months later.
Hoping not to stretch the analogy too far, I just came back from a 95k bike ride today (another newly learned skill in my old age) and despite the physical fatigue, if I am able to be stay mentally sharp, it's amazing how effectively I can put out the watts as long as I relax and remember to cycle smoothly in relaxed circles.