How do I know what my max is (so that I can then calculate (?) the range above)?
A rule of thumb is 220 minus your age, but that can be inaccurate depending on your fitness level. There are also calculators online that take other factors such as your age or gender into accounts, again being an estimate.
Empirically, run up a hill or sprint hard as fast and as long as possible. That will give you a good number. Wear a chest heart rate monitor, these tend to react faster to your changing heart rate and typically give more accurate results than wrist heart rate monitors.
For context, I'm physically skinny at 5'10", 135-140 lbs. I trained to run a 5km in 40 minutes. Walking would have been faster, but I promised myself to maintain runners form the whole time which is more draining going slow.
Tried out doors, treadmills, air conditioning, music, youtube, basic programs, running guides, etc etc. Watching video game streams generally helps my mind best from how painful, annoying and how much I want to stop doing what I'm doing right at the moment. And this moving at 6.5 km/hr for 25 minutes. If I had a break in my routine, it takes me about 4-5 weeks to work up to at 3-4x/week.
It's all terrible.
I have high blood pressure so exercise is a requirement for me. I'll keep doing it regardless.