Sandi, I don't know if I can be of much help, but as there are no other replies I thought I might as well answer your question! I had a Baker's Cyst which was a bit uncomfortable but didn't prevent me from running. I went for a scan (not an MRI) privately at my physio's suggestion after he diagnosed it, When I got there, I was told that the cyst had already ruptured, so no further treatment was needed. I do wonder if running on it actually helped rupture it... Yours case sounds worse than mine though. Let me know how you get on.
The Royal Parks Half Marathon is one of those which isn't registered - or it wasn't when I did it last year. It seems odd for such a major event but it's always over-subscribed and I suppose a lot of the entrants aren't too bothered. I only found out because I ran a PB and it would have been nice to have it included in the rankings.
I saw a consultant the other day about an arthroscopy and he told me it would take six to eight weeks to return to running or cycling. He said it would more likely be eight for the average person, six for elites who have more facilities to help them recover.