I tore my ACL and injured my MCL in a skiing accident about 7 years ago. I was not a cyclist then. I had the ACL surgically repaired and the MCL did not need it. I did all the physical therapy, but it was a really hard and long recovery. A person in their 50's does not heal as fast as a person in their 20's. I still did not have full range of motion at either end.

Then I started cycling. The cycling gave me back the full range of motion. I don't think I would be comfortable without having repaired the tear because I would never know that if I just moved the wrong way, I could re-injure myself, and I don't want to go through this again. If I was you, I would take care of the knee and postpone the event till next year. The surgery is done arthroscopically. I know that's not what you want to do, but it might be better in the long run. And cycling can be part of your recovery! That's why I use half toe clips and am deathly afraid of clipless pedals....