This would be a fun one to work through. If 100 hours of working out reduced your heart rate by 1bpm (I have no idea if this is true, just sounds plausible), but during workouts you increased your BPM by 20%, where is the equilibrium?
You also will want to model the fact that your resting heart rate does not stay at a lower level forever after exercise.
Finally, but that's peanuts compared to the other problems: let's start with a starting rest rate of 60 BPM. Exercise for 1000 hours to bring it back to 50 BPM, and your heart rate during workouts becomes 1.2 x 50 BPM = 60 BPM? Unlikely.