So, my method personally has been just spending a lot of time on the stair master some days to strengthen my legs + when I run on the treadmill, I just keep going the same pace for days on end then take a few days break to recover and then just increase .5 speed after 2 weeks of the same pace. At least that's how I worked up to 7MPH.
Then from there it just became a mental game, telling myself "If you run faster, you can get done with this mile faster and then you don't have to run anymore, if you run slower, this will take forever". But as of right now, I've been trying to find the right way to get over the 1.5-mile hurdle. 7MPH up to the 1.0-mile mark is difficult on its own, but as of right now I need to push myself to break past my limit and last just another 4 minutes and 28 seconds which on paper doesn't sound too bad.... but in physical reality feels like forever.