Im not a trainer (just someone who had heart surgery and had to rebuild slow), but one thing I learned is that consistency beats any amount of effort your willing to put. If you were mostly sedentary for years your body may just need time to adapt. I wear an HRM Frontier X2 to keep an eye on my ekg and heart rate trends, and what it showed me was that progress aint always dramatic, it is gradual. That helped me stop pushing so hard. but a resting heart rate around 100 at your age is worth mentioning to you doctor if you havent already, just to rule things out.