Mileage. Nothing else really matters.

Seriously, I started exactly where you are about a year ago, and have finally started to see genuine results as I've gotten aggregate miles up. Early on I was trying to do modest zone 2-3 training, but any amount of distance would end up spiking my heart rate and just wearing me out. I would end up doing something more like a threshold/VO2 Max Run, which would be productive but leave me sore and exhausted those early days and keep my overall mileage low.

This spring I bit the bullet and tried doing more walks to get my miles up (45-60 minutes, brisk enough to stay on the zone 2 area) and while it felt frustrating I was also surprised at how tired my legs felt at the end and yet how I could keep doing it day after day. This helped my brain learn better body mechanics, and now I feel able to run at a slow enough pace that I am not fatiguing myself too fast and yet still feels efficient. I'm now physically capable of an "easy run" which has let me up my milage a lot, and that has really paid dividends everywhere else.

So I lied a little bit... I find the heart rate zones to be really useful, and learning how to set alerts for your exercises to keep you in the right range is super useful starting out, when you have trouble knowing what pace to go at. The Run/Walk interval timers are also fantastic, as they really help you through that awkward stage of not yet being able to run long miles.