I am assuming that you are running on the treadmill?
If you only do one form of exercise, you will end up with a fairly narrow base of fitness - in this case you wouldn't be doing much to build endurance. In terms of building a broader base of fitness, you probably want to keep your HIIT at 3 days per week, and try some different things on the other days. A tempo run might be one idea - basically about 30 mins fast, as well as a long slow run once a week or so.
Also, you may want to cross train with a different form of exercise entirely, whether cycling, the elliptical, an exercise DVD or whatever. Not only does 'different' give you a broader base of fitnes, but 'different' will probably also help with boredom.
The reasonable man adapts himself to the world; the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man.
| current weight: 178.0