I am so sorry to hear about this. It's hard when you've got the motivation to exercise but something like this interferes.
BTW, do you know if it's a bone spur? Did your doctor suggest anti-inflammatory drugs or orthotics?
About cardio, that's so tough. I wonder if recumbent biking would work for you. If you have access to water, you could do laps or do them with weights and a kickboard...? Maybe there's water aerobics available to you somewhere?
One thing that you might consider--and this will sound VERY strange--is maybe using crutches and then going for cardio. When I tore a calf muscle and was on crutches, I used to do fast-laps with crutches around the gym. It was a challenging workout.