I never thought I'd be doing this much cardio either. But I started at 2 20 min sessions every day. I've got a Precor at home, so it is actually doable because my gym is such that it would take me an extra hour / day just to get there & get to work. My trainer originally recommended 2 sessions at a slow but steady pace. I did this for a while, but I just think HIIT works better for me. I usually do 20-30 min of warm-up & HIIT and then the remaining time at a steady pace in the AM. My night work after training is usually 30 min on the stairmaster & 20+ on the treadmill at a steady pace.
But the secret has really been starting low and building up because it is all about conditioning.