Heart rate ?

I does stabilize a lot as you get fitter; overall the healthy heart rate depends on your age and general frame (for humans there's not that much variance but for example an elephant's resting heart rate is 28 and a mouse's 500 beats per minute); so basicaly a teenager will have a faster heart rate than a fully grown person.

Anyways, when I first started working out I was really out of shape, I'd get up to 180-185 and stay there which supposedly isn't that healthy but it didn't feel any differentfrom 170 and I felt like I could still push myself. Now though, it generally stays between 160 to 180 even when I'm really really pushing (intervals).

That sounds like your body learned to perform in that range. How long did it approx take you to have a healthier range permanently?

The biggest breakthrough was interval training: before that I'd kind of crawl very slowly up the stamina incline but with intervals it stabilized within about a month.

Though, for the past maybe... 3-4 months I've been doing very minimal pure cardio, just enough to warm up and then the rest of the workout is strength training (but, with max 35 second breaks inbetween sets, so it keeps my heartrate up) and I'm seeing that the cardio heartrate has not suffered at all, if anything it's gotten better.

Edited to add: my brother is one of those annoyingly fit people, he's always been into sports and has a degree in physiotherapy; according to him one of the things that indicates how "in shape" you are is not only how high (or rather, how low) your heartrate spikes when you're really pushing it but also how fast it declines to normal again once you stop pushing. That's probably why the intervals are so effective: they repeatedly force your heartrate up, then down, then up then down.

I have an issue with this and have a family history of heart problems so I play it safe and Walk at 3.0 mph at an incline of 3.0. If I feel like I can push myself more I either up the incline or up the speed. Usually I end up rotating going either faster or having a higher incline every few minutes and I check my heart rate every few minutes as well.