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.