I can’t improve my endurance

Patient: I’m reasonably fit but following a serious chest infection 1.5yrs ago I can’t seem to jog more than a mile. I could jog 7-10 miles non-stop before. Is it my heart or lungs?

Symptoms: Low maximum heart rate – 80bpm (not the norm for me)
Low resting heart rate – 35bpm (normal for me)
Lack of endurance when exercising
Short bursts are still ok
Sinus issues recurring – not sure if related.

Doctor: