I was a mid-17s guy in high school off decent mileage for a high schooler (up to 75 miles/week), so I think I fit the OP's criteria of not studly but average runner.
Of course, this being letsrun, people have a distorted impression of what is "average". But I digress.
I kept on running mid-17s into my early 20s with 60-75 mile weeks, and then at about age 23 had an epiphany, time on my hands, and a marathon to train for. Bumped weekly miles up to 80, then 90, then 100 and ran 16:09, 33:51 and 1:16:42, latter at the end of a track season where I kept the mileage around 80 (100 in base phase before that).
Did lots of long runs throughout the year leading into the half. Staple workouts included 40 minute tempo at about half marathon pace or a bit slower. Now that I'm older I realize that a 60 minute run at marathon pace would be a good one to add to the repotoire for half training as it gives you practice in holding a solid pace for a long time.
If you want to run a good half you have to work the 3k-10k paces as well, and I would typically do that in sessions of 4 minutes at 10km pace, 2 minutes rest, 2 x 2 minutes at 5km pace and repeat that 3 times. Throw in some strides and 300s at 3km pace every 2-3 weeks to work the quicker paces.
The long and the short of it, is to run a 1:15:30 half you need to be comfortable at 5:45 pace. So spend a lot of time running at 5:45 pace. It should feel pretty comfortable for 20-30 minutes. Then stop. Do it again in a couple of weeks, and in the alternate week make your tempo a longer (40-60 minute) effort at a slightly slower pace.