looking for training advice for sub 135 half

did the downhill mile have an undefined slope?

Progressively work to double your mileage (if that is your typical mileage above). Then add goal pace work and progressions runs. It will be a done deal.

I ran 1:36 pace for 12 miles in my long run last week. Does this mean I've transcended hobby jogger status now??

How long should my progression runs be? Also, is there a certain speed workout I should incorporate into the mix?

8-12 miles, depending on your total mileage. Speed workouts? Check out the other thread re: cruise intervals at threshold pace.

I would say 20-25 miles is fairly low for a 1:35 goal. Assuming you're doing a long run of 10+, that leaves only 15 miles spread out over 6 days. I would increase that, add in a few more 40 minute runs on other days.

As other folk have said, tempo workouts are (in my own experience) the best way to improve half marathon times. I would look up threads here, as well as reading Daniels. Daniels will give you pace guidelines as well as good examples of tempo/long run workouts - 2 miles easy, 4x10 minutes at tempo, 2 miles easy - That type of thing. A decent long run, tempo runs, and a few more miles you'll have no trouble getting sub 1:35.

Do you even need to train to 135 for a half marathon? I mean, just jog that shit. Are you an 80 year old midget?