I suggest you follow the Buffalo protocol. Check your heart rate and exercise to increase it 20%. If you have no increase in symptoms at that rate, even delayed increase in symptoms, increase your heart rate after a few days. Step up your target heart rate each week. If you cause an increase in symptoms, back off 10% and see if you can exercise at that heart rate. After a few weeks at that target rate, try increasing your target rate again.

You may find a heart rate that you can maintain for extended periods. That is better than pushing to a symptom level.