Like a few people I tore my ACL. But I also managed to tear my MCL and Pateller. Once all the swelling had gone and I was able to ride a regular road bike again I was told to switch to a fixxie with a easy gear. I rode a 40/16. (69" I think) Spend hours and hours just riding around, Commute on it too. Do as much riding on your fixxie a you can. Dont worry about "perfecting" your spin. It will come. Without knowing it you will build an amazing set of stability muscles in your legs. A good chunk of the time I would ride with guys on theire road bikes and just spin my brains out. I still do that. Funny how what was a "rehab" from a bad downhill crash turned me into a trackie.....

The 2 biggest things again are 1. Light gears! 2. Get your saddle position absolutly perfect.