I took the plunge in the middle of January and have been using my local fitness center about 5 days a week, for approximately an hour each time. We are cuddling up to the middle of March as I write this so I thought I would share my thoughts on the results of my little experiment.
The most significant change is that I've lost weight. Yea! Also, and perhaps more important, my back problems—I was experiencing quite a bit of pain in my lower back—have disappeared. AND when I run for the bus I no longer appear to be at death's door. All of these things are wonderful and, collectively, they have made me feel much younger.
To come back to my original question: Is it worth it? Is the time spent exercising worth the lost writing time? As you've probably guessed, I don't think there is a clear cut answer to that question but I can say that it was worth it for me. I am healthier, and probably happier, because of having started an exercise program and I can only hope that is reflected, in some way, in the quality of my writing.