WOW! I'm impressed!
When I was in rehab, they didn't decide I was ready for jogging until almost the end of the 12-week class. Like you, I was no speed demon, but I felt better jogging than walking.
Fast forward a few years, and I no longer jog. My heart is well up to the task, but my knees just can't take the pounding. (Guess that's what 30+ years of running can do.) So, I fulfill my cardio-training needs by riding a stationary bike (16+ MPH) or power-walking on an inclined treadmill. My objective is to spend at least 30 minutes daily with my heart rate up in the 130's to 140's (beta blocker gets involved here) - and these exercises do just fine.
Keep up the effort. Remember that in rehab, you get out of it just what you put into it. Push things and you'll go farther in the long run. Just don't over-do it.