Tae Bo..

I was getting bored with walking, and jogging up stairs, and typical situps so I decided to try Billy Blanks Cardio inferno tae bo and the workout is 51 minutes. I was only able to get 13 minutes into it before my arches in my feet started to hurt so bad I was in nearly in tears! I know I need new shoes, but until I get the money I'm just going to have to deal with the ones I have. I was just curious as someone who has pretty much never really worked out in her entire life...is 13 minutes of tae bo okay? I know it's not a lot, but I also have bad, bad knees because of them being dislocated more than once on each side. So I am trying to ease into the more complicated workout stuff. I guess my issue is, if I don't do the entire thing I feel like a huge failure. :| So this topic is sorta bouncing all over, but my main question is: Is starting out with what I can do and working up to it alright?