i've been injured numerous times and have had to lose anywhere from 2 weeks to 3 months of running on several occasions. i've always found that keeping track of total time cross training is key, don't worry about converting it to running mileage b/c cross training isn't running. unless you were in horrible shape previously, i doubt you can GAIN fitness, but you certainly can maintain 100% aerobic fitness. the only things that may hurt you are missing workouts and possibly an altered gait if you don't get in a few runs before your next race. just make sure you cross train enough and get your heart rate up!
for some inspiration, i had a hamstring injury last winter that made it impossible for me to run for 3 months. i didn't have access to a pool but i biked about 4 hours/week and used the elliptical for about 3 hours/week. once i got back running again i let myself run easy for about 6 weeks (i was in no rush) then once i was doing at least 30mpw i did a 1600 and 5000 time trial, both alone on a track. i was within 13 sec of my previous 1600 PR (with NO speedwork!) and within 9 sec of my previous 5k PR (although my previous 5k PR was not incredibly impressive).
just cross train like a beast, try different cross training methods (pool, elliptical, bike, jump rope, etc.), and don't forget the essentials (a long "run" can be done on a bike or elliptical easily, although it's even more boring).