I would focus on getting used to running 3-4 days a week OR ~9-12 miles per week before beginning the training program. Even though you say you've run 5ks before, you need to get a good base level fitness before you start those long runs on the weekend. Once you have that good base fitness training for a HM in 12 weeks is doable. If you can give yourself a couple extra weeks at the beginning of the training program then that helps a LOT too.
Just remember, these programs are guidelines for training, but you need to listen to your body. Working out 5 days a week is a lot especially if you're not already used to it - I think I stuck to 4 days a week for HM training and walked the dogs ~1 mile on the other days. I also gave myself more step back weeks, where I cut the mileage way down for one week to let my body recover. Check out some other training programs such as Hal Higdon's programs, or programs on Runner's World.
Good luck!