Yep, I'm on the long side too. Working out yesterday with a guy ~half my age doing leg extensions. We rotated sets and the only "rest" we had was the time the other guy took to complete a set, and then add or delete 45# plates on the machine. That was one of the best cardiac workouts I'd had in a while.

Interval training, cross-training, circuit training -- all good. My days of racking up miles damaging my knees is way behind me. Legs are much stronger for it.