Other Stuff To Do...
Take a moderate to brisk walk, hike, do some yoga/stretching work, or otherwise "play" a little one day a week, and consider taking it real easy one day a week (if you're moderately to intensely active for 6 out of 7 days). If your workouts are burning you out and/or if you're intervals are leaving you drained, you're probably either doing too much too soon (in which case you'll want to increase gradually) or you're doing too much, period. Planned rest is a key component to fitness. So, do your intervals 1-2 times per week (perhaps prior to your longer, more moderate workout on the treadmill/bike/pool/track as set forth in my post: Get the most out of your cardio machine or "steady state" workout!). Add in some extra cardio time (if that's your love) on other days, make time for at least 2 days a week of strength training, and make sure to take time to stretch and REST!
Speaking of playtime, here's some cool stuff to try on one of your mild or resting days! ... apparently it'll keep you young!