My only concern would be that it'll cost you another $55 to get the second disc by itself, while you could bundle it with the first disc in the combo pack for just $15 more than the first disc alone - that's actually a pretty good value. The disc 3 and 4 workouts are very good and nice to mix in for variety but Fat Burner/Below the Belt/Diamond Cutter/Red Hot Core really are your go-to workouts for at least the first month or two. Not counting 'wake up' which is just a nice morning stretchy thing but not at all critical. Working the intermediate program, I don't think I mixed in anything off discs 3 and 4 until like week 10. Looking at the workouts that are on disc 1 versus everything else, you can definitely start with just that. I have the combo pack - actually the big pack which includes extreme and whatnot.
Pretty much all of these are going to get your heart rate in the fat burning or cardio zone (maybe not energy for too long if your fitness level is already super high, which it might be as a runner). Maybe worth mentioning, fat burner isn't the only one that will burn fat/get you breathing for cardio.
