[gavanwoolery]

I was mostly sedentary until about 1.5 years ago.

I have tried almost every form of exercise, but the only thing that has truly hooked me so far is circuit/interval training (for both cardio and resistance). I have done almost 400 sessions so far, almost without fail at 5 days per week (which is the real testament to how addicting it is).

Actually I find the cardio classes to be more interesting and challenging than the resistance / weight lifting classes. Although the focus is cardio, they do not neglect training your muscles, they just tend to rely more on body weight than barbells (burpees, jump squats, sprawls, etc) with the occasionally machine thrown in (rowers, bikes).

I have not lost much weight (that requires a better diet) or gained a huge amount of muscle but my body feels way more tuned. The classes are pretty intense and burning 1000 calories/hour is common. My heart rate used to max out at 180 and my resting heart rate was 80-90. Now my max heart rate is 160 and my resting gets as low as 50. I used to struggle to do 50 push-ups now I can do 100 without much struggle.

The real benefit though is that I feel healthy and less afraid of things like heart disease.

[nradov]

Congratulations on your progress, but those numbers don't look quite right. Most people can't sustain 1000 kcal/hr for a full hour, especially not when doing circuit training. Consumer fitness tracker devices commonly overestimate calorie consumption by a significant amount.

Max heart rate doesn't really decrease with fitness unless there's some weird pathology going on. As fitness increases you can generate more power at the same heart rate, but max heart rate remains roughly the same.