Not sure if this is absolutely good advice, but i cut off my own cast to under the knee at four weeks and dealt with the pain. I didn't want my knee to get all stiff. it was REALLY painful for a few days and then all of a sudden - the pain lifted for my knee and i started doing my own therapy. The risk to all this is that you might fall and injure yourself again. I was super careful and didn't go very many places (although i think i tripped over something - but it got my knee more functional because it could only bend about 1/4 of the way at first). The cast I had was thigh high, too.
I sat on a chair and did these knee bending exercises over and over. Just little bits at first and then more and more. Then, as soon as I got my cast off - i started walking around a lot. Then cycling. Now, it's really ok unless it's about to rain. It's like a barometer.
Pedaling is something that comes back to you - so it's not something to get excessively worried about. I agree with invictious about spinal problems however. You don't want to twist too much. I had a piano bench to the side - and put my leg at a 45 degree angle at first and didn't pedal at all. Then, when the cast came off - pedalling was sore and difficult. It does get better.