I prefer to keep my workouts to 45-60 min because of time constraints, but during my training season, I have no choice but to run 2, 3, sometimes 5+ hours on a long run (my 20-miler is this weekend, so that'll probably take at least 5 hours). During my off-season, I get to stick to a more manageable schedule of 45-60 min.
This is one reason I haven't been able to use a lot of Cathe's later workouts; I always feel as though I'm cheating somehow if I do a pre-mix (I know, silly), but I just don't have the time (nor the energy!) to work out for longer than an hour, especially when I've just finished 5 months of grueling marathon or half-marathon training.