You and I are in very similar situations! I have a lot of back problems and an ankle that currently looks like it belongs to an elephant. I, like a lot of people here, managed to lose a lot of weight about five years ago and did it mostly through walking and added in the elliptical machine later. Stomach was the Captain's Chair. This time around, sigh, I have added in Dance Dance Revolution!! I am not kidding! Last night I meant to only do it for 30 minutes... 60 minutes and a leg cramp later I finally stopped. I really enjoy it. I am terrible at it and have to be careful with my back and ankle but I still get a really good workout in and have fun at the same time. Who knew?
I also try not to focus on what I use to be able to do compared to what I can manage now but am trying to focus on my, gasp, former love of exercise. I distinctly remember enjoying waking up in the morning and stretching my legs so I could feel all those cool muscles. I have an 11 month old now so those muscles will take a little longer to get to than then but we can do it!

Perhaps a goal? I walked a half marathon and did a 100 mile bike ride for charity at about 230 pounds, it can be done! Plus, when you are doing it in part for others, it adds some great motivation!
As for your injuries, I am sure you already know this but ice is your best friend, even though heat feels sooo much better. If you can get in with a good physical therapist even once to give you exercises to do at home that may be a big help as well! If I can get a little personal, a great bra has helped a lot with my back as well. I use an Enell which is expensive but fabulous.
I ramble but I hope that helps... you are not alone and you can do it!