The program I am in has me exercise now until my heart rate is >110 and having symptoms. Symptoms are graded 1-10 (10 being worst) If my symptoms are 0-1 but only go up 1-2 pts then I keep going. If they go up more than two points OR symptoms last for too long (>20-30 min) after exercise it is too much. So shave off not time but intensity. Maybe 1/2 at full and 1/2 at easy pace. My PT had me start at 10 minutes on treadmill 2.2- speed, slowly increasing incline to level 3 over 10 minutes then last 10 minutes if OK at 0 incline and less speed.