Just to add to the list of people who are incredibly jealous of your 5K time zombie :) Good luck with extending the training.
I've done a few half marathons in the past on 3 or 4 runs a week, but with other stuff in between (swimming and cycling mostly, sometimes gym), so don't discount carrying on with other things if you enjoy them.
The key to my training when I used to get (relatively) decent times seemed to be a decent long run once per week (although I never used to go above 10 miles or so beforehand), and long reps for any "speed" work.
So I used to run to the seafront (I grew up by the sea) and then do 2 or 3 1-mile reps at a decent pace, with a minute or two rest in between. Or I'd do 4 or 5 800m reps. And when I did hill runs I'd find a loop with a long-ish hill and run up and round it however many times...
It is so hot out there. And I'm still dragging a lot of fat around with me everywhere right now, which isn't helping!
I ran at 9am yesterday 5madthings. It was actually okay, and we headed straight for the woods!
That was going to be my suggestion sunflower : just head for somewhere shady... but that's not a lot of help if you're in the middle of a city. We're fairly lucky here that there's lots of trees we can head for!
Hamstring is definitely not quite back to normal, but it's not hurting today after my run yesterday, so am hoping to head out for a shuffle round the block some time tomorrow....