Don't wait to be feeling your best. Even on the days when you're tired, sore and grumpy, get up and run through your routine. Building a habit takes time and repitition. Every time you take a break, imagine yourself putting a break in a chain. No matter how long you've been following your routine, all that past work and discipline goes to waste if the break you take is significant enough to break the chain.
Obviously, on days when you're feeling ill enough to stay home from work, you can skip the exercise. But other than that, you should be getting up and running through the motions even if your head is not in it. If you wait until you're in a great mood, or the ideal time, you will not be getting the regular exercise and benefits you deserve.