First thing, go get a blood workup done to make sure that you don't have any issues. Have a CMP with glucose tolerance along with T3/T4, and testosterone. If any of them need adjusted or you've got issues with glucose, then you should do whatever you need to do. If your testosterone is low, you should get it adjusted upwards. If you're currently on BP meds, and cholesterol meds, once you begin AAS / HRT, you should immediately see improvements in your BP, lean body mass, and cholesterol. Much of the aging process is that we don't adjust our hormones properly upwards to slow it down. Sooner is better than later. Hormonal adjustment (AAS, GH, T3/T4) along with diet and exercise, even when begun in middle age, can add years to your life, improve the qualities of that life, and prevent a long list of diseases.
Remember to begin slowly, and not to be a "weekend warrior". For improving your cardiac threshold, and fat burn, cardio interval training is superior. Don't listen to quacks at the gym who have no clue, and who haven't dug into research and truth.
Whether you're 13, or 85, you can expect a remarkable change in your overall fitness and strength in the first 18 months of training.
Resistance training is the ONLY exercise that strengthens your bones, hypertrophies your muscles, and improves your cardiac threshold AS YOU GROW OLDER. With proper hormonal manipulation, you can do very well into your 70's and beyond. The important thing is to begin NOW, and to integrate it into a healthy life style of diet, exercise, rest, and proper hormone therapy. There's absolutely no reason to be over-medicated, and feeling crappy in middle age.