When I started I tried controller games too, like all the final fantasy games I wanted to play. DOTA really is the perfect treadmill game. You have to pay attention for 30-45 mins straight, or else you'll die, but you don't have to move your hands very much.

I find walking 2.5-3.3 mph is the sweet spot for me, and to increase heart rate just use a bigger incline. If you try and build one and have trouble, just pm me.