Heart pounding during moderate exercise

I'm 52 y/o male.  I used to run 3 miles x 5 days per week.  The last two years I have not exercised.  I recently took up boot camp training.  Upon exercise (sled pulling, tire flipping, walking lunges...etc) my heart pounds very hard.  To the extent I have to sit and rest.  I do not experience chest pain, just severe heart pounding.  Is this due to being out of shape for the last two years?  I do take synthroid, statin drug, and BP medication.

[CCF HeartMD6]

I am somewhat unclear by what you mean by "heart pounding." However, if you are experiencing new symptoms with exertion not previously present before -- SOB, dizziness, light-headedness, or fainting, I would suggest a routine follow-up with your regular physician for follow up with your thyroid condition as well as evaluation of your cardiac risk factors (blood pressure, lipids, etc).