I'm on the treadmill pretty much everyday, trying to lose weight and increase cardio fitness. I do the "weight loss" program, which essentially goes: 2 mins warmup, a minute walking flat, 2 minutes walking uphill, a minute walking flat, a minute fast *repeat for duration of program 2 mins warmdown I'm currently doing level 2 on this program. This gets my heart rate above 135 for most of the time. I'm currently doing 24 minutes on the treadmill. Question is: it's time to either lengthen the time, or increase the level (which I think just increases the steepness of the incline in the "climb" parts) ... which should I do, considering my goals?