Well, I'm not the authority on the subject, but I'll give you my 2 cents. I think you just have to take baby steps. You may not have time or energy enough for a full tae bo workout, so take a 15 minute walk. Do that every day this week, then take it up to 20 minutes next week, and so on. You might give your knees a chance to build up before trying something a little more challenging.
When I first started coming here to 3FC, I had talked about how busy I am, how there is no time for me, etc. I don't remember who it was, but they basically asked me what commitment I had to making this work. She went on to say that if it is important enough, I would find time to make it happen. I think I needed someone to slap down the excuses I had and make me face up to the work I needed to put into it.
Then, come back to 3FC often to read and post. You will get the support you need. It's a great place!
End of 2014 Challenge:
This time, I'm going to be stronger, I'm not giving in. - Rudimental