I've given up trying to predict these things since so much of it depends on individual genetics, ability, current fitness, etc. If it were me though, I'd race a 5k or better yet a 10k as soon a possible to figure out exactly where my run fitness was at. Then if you absolutely can't run 5x/week, I'd try really hard to push one of those short runs to 45-60 minutes, and make it a tempo run using whatever Jack Daniels spits out based on your recent race result. Good luck!

nah. I'd imagine high 1.30s.

Read Matt Fitzgerald's 80/20 book (80% easy, 20% fast). You can get quick running easy, low HR stuff.

I found what took me from 1:40 to a PR of 1:32 for a half was a three month cycle of consistent once-per-week speedwork. I was really surprised that doing such short intervals could take 8 minutes off my half time, but its been really effective. Once a week I would do a tough speedwork workout on Tuesday mornings, with 2-3 miles to warm up and another couple miles for a cool down. Distances varying from 400m to 1200m for a total of 4800m per workout (i.e. 12x400, 6x800s, or ladders from 200-1200m). I would aim to do my 400s at 5:40-6:00 pace. Two other runs during the week would be 3-5 miles relatively easy, and then one long weekend run (7-9 miles) for a total of 20ish miles per week. I'm 31, and 6'1" 185lbs, like you. I think you have enough time to at least come close to your goal, since your race is at the end of May. Note-if you're not used to speedwork, take the first few weeks a bit easy, or there's a good chance you'll pull/strain something or re-aggravate your shin splints.