I was an 800/1500 runner at a D3 college with the occasional 4x4 and had to do something similar due to injuries. My week went something like:
Monday - Workout, usually on the track
Tuesday - 80 minute elliptical
Wednesday - Workout
Thursday - 80 minute elliptical
Friday - 30 minute bike or elliptical + 10 minute run + strides
Saturday - Race
Sunday - 120 minute elliptical
So that's just over 5 hours cross training with all my runs quality (I think I was usually around 35-40ish miles per week). I started doing this mostly Junior year and took about 2-3 seconds off my 400, 5 seconds off my 800, and 10 seconds off my 1500 by the time I graduated. I was never that good to begin with so those numbers sound bigger than they are, but I definitely improved. I even won "most improved athlete" on the team after Junior year.
Post-college I've been running more and cross training less and I'm in better shape now, however, especially for longer distances. There really isn't a substitute for running miles, but cross training is the next best thing.