The heart rate
response is actually suboptimal, given that according to your age what we would expect is a pulse of around 180 with peak exercise (220 minus age), and thus it may be that you are still deconditioned and not at top cardiovascular shape. That could be the reason why you are short of breath with lower than expected heart rates. I would advise to continue a gradual, aerobic exercise
program prescribed by your doctor and follow up if there are other worrisome symptoms or limitation of activity.
Dr Brenes-Salazar MD
Mayo Clinic MN