How hard to go from 1:30 to 1:25 HM

Ummmm...easy. Just run baby.

If you're new to running, the gains come quickly at first. Without doing anything other than jogging 30 miles a week, my first half marathons (about 6 weeks apart) went:

1:31

1:27

1:22

1:21

1:20

1:18

1:18

then, I plateaued until I joined a club and upped my training

eventually got to 1:10

good luck

What were your PRs & mileage like from 10 years ago? Faster than current?

That 5k is a good bit stronger than the 10k & shows 1:26-1:27 potential. I agree that consistent running should get you pretty close. You're running well off of less than a year of structured training. You do need a little more leg speed. I'd include some VO2 work early in the build. Strides 2-3 times/week. Some volume 200-400 workouts. I also think sub-1:25 can take slightly higher mileage for some (50-65). A little speed work to start, a slight increase in mileage, and then a flurry of threshold workouts is the recipe I'd go with.

Looks like a 2% improvement in your 5k time would put you at 1:26 or high 1:25 for the half. I don't ever shoot for more than 2% improvement from each training plan. Shoot for 1:27 and adjust every 4 weeks or so depending on how things are going.

PRs from around 15 years ago were 17:30 5k. Focused on 800m in college - 2:00 around that time as well

Where on Earth did you hear this? This is the most ridiculous training rule I've ever heard of.

Seems doable to make improvements off of consistency/added mileage. 17:30 is plenty fast enough for sub-1:25 if you can get down closer to that.

Sky is the limit with the 800 speed. You see people running 70-72/sub-2:30 off of that.