Hi DBS, don't be scared.
Try to improve on your stamina, it will train your heart too, and it becomes more efficient, and stronger and in the long run also bigger. As the capacity of this bloodpump rises, your pulsrate drops.
Eat a bit healthier, and less (if you think you eat too much) drink more water.
For someone your posture it's probably better to cycle or swim instead of running, becaus this way you won't wear your joints (feed, enkles, knees, hips, back etc.)
Start out slowly and progress slowly, as a body needs time to addapt. A common mistake is to 'build up' to fast. Resting days are the days where recover, the night/day after that your body builds the extra's.
Happy training and freediving!
Kars.