I actually started by walking. Maybe 30-minutes every other day, outside during lunch. When I returned to my bike, my wonderful spouse rode behind me (as I was nervous about it). It was 12-miles, but I had to stop and walk my bike at time. Now I am zooming through the 12-mile ride and cruising up all the hills! Also, my pulmo (he’s a good one that works with Covid Survivor Corps) recommended breathing exercises with resistance bands. Basically you inhale deeply during the resistance, try some “breath-stacking”, hold for 4-seconds, the breath out very slowly as much as possible. I swear this has helped (in fact I just finished … the routine takes about 30-minutes). You may want to ask Mayo about some kind of rehab program.