Do you see my point? Anything is possible to those who believe. There's a famous saying; "He who says he can and he who says he can't are both usually right" -Confucious. This indicates that WHATEVER you set out to achieve, as long as you're committed, and you believe in yourself, it IS within your grasp.
Motvation is absolutely necessary for an undertaking like the RH Challenge. It's not always a struggle, but there will be times when you've got to dig deep. At least at one point in every workout, the fatigue will set in. You'll feel the sweat dripping down your face and the lactic acid draining your strength when you know you've got 4 reps left. Do you give up? NO. GET MOTIVATED!
The catalyst that motivates you has to come from deep inside. Whatever your reason is for taking the challenge, whether it's to lose weight or something else, you've go to find it. After you find it, harness it, then channel it, and don't quit until you hit the end of day 21.