The only form of strenuous indoor exercise I really like is using an elliptical machine, which is ghastly boring compared to Gotham Breakneck outdoors. I can watch a movie or TV show on Netflix, but the kind I prefer for entertainment — at a thoughtful speed, usually — lose my interest when I'm exercising. Only dramas filled with action or, better still, fast-moving deceit and intrigue can hold my attention. And there are only so many of those that are good enough to watch.
So I've been watching math videos, some of which are really wonderful. I find they stimulate me to considerable exertion on the machine. The problem is that they're rarely as long as my 40-60 minute elliptical sessions. Some are as short as 10 minutes.
Using QuickTime Pro, I can combine any number of .m4v files into one and save the result with no loss of quality. This has completely solved the problem.
I have v. 7.7, which I am using on Mac OS 10.5.8, an installation two operating-system versions old but totally adequate to my needs.