If walking is what your body can handle (due to back pain or whatever) find a hill and walk quickly up it.

"walking" may not be cardio, but I hike uphill for an hour and depending on my speed (I don't run), my heart rate can get way over 200 if I want. I also use hiking poles for total body movement and increased energy expenditure.

Don't feel you have to do something high impact to get the heart rate up.

take care,
Meg