...and by the way, the fact that you can do high-intensity cardio is a good sign. I could barely do 100 bpm in the first month post injury, and it took maybe three months to work my way up to 120 bpm. (I can now exercise for a half hour at 145 bpm, though I feel it somewhat the next day).

But do make sure that cardio isn't aggravating your symptoms. I believe the rule is that you've hit your threshold heart rate if your symptoms increase by 3 points on a 10-point scale. And once you've found what this is, you should be exercising at 80% of that.

If exercising isn't really aggravating your symptoms, I would even crank it up and do more than 20 mins per day.