You can run if you have a cold, but nothing too exerting. Don't push a 3 mile run if you're sick.
In class today I was asked a good question.. "Are you injured? Or are you hurt?" I took it to mean that unless I'm injured, I need to keep pushing. Training to fight, you'll be in a perpetual state of hurt.
Back to your question, unless you can get 8-10 hours of sleep EVERY night, I suggest letting your class be your cardio. If you can get that many hours of sleep, you need to start doing roadwork. Go to
www.mapmyrun.com and plot out a course in your neighborhood.
I would also do sprint. I like to do mine on a treadmill. I run for about 4-5 minutes at a 8min. / mile pace and then I sprint for 20sec. on/ 10sec. off for about 5 minutes, then back to running at 8min. pace, then cooldown. Great for those bursts and, personally, I am finding my cardio isn't holding me back in training, just my injuries.