Just like you said above, a nice thing with using cardio equipment is you can really push yourself that much harder because you can provide yourself with the exact challenge ie. level 5, level 7, or even level 10. Sometimes when you are running or biking outside, while you may be working out hard, you never know exactly what level you are going at. I suppose if you were to have a heart rate monitor that would help.

In the end, I find the toughest workouts are the ones where based on the numbers I push on the bike or treadmill, I am committed to doing that program until it is done.