OK, if you can currently run for about an hour at 4:55/km 'relatively comfortably' that will be close to your tempo pace. So you first build up your existing runs to run every day at 5:30-6:00/km for the hour (or less if that include getting ready) that you have available, adding extra days before you add extra distance. When you're up to running for the max time you have available each week you do that for another 3-6 weeks, then start to do 1-3 of those runs each week as tempo runs by running 20-40mins at tempo pace (4:55/km) after a warmup. For half-marathon you will need to add a distance run, starting at 1hr10 and adding 10mins each week until you're running for 1:40-1:45 at an easy pace. After 3-6 weeks of this tempo phase you're ready to replace 1 or 2 tempo workouts a week with intervals at 10k-pace (10sec/k faster than tempo) & 5k-pace (20sec/k faster than tempo). 3-6weeks of that and you'll be ready to blast your half-marathon. Strength training & stretching help to prevent overuse injuries, which is important because it is the consistency of training that will result in optimum improvement, not the intensity of any one workout. So you may need to make recovery runs a bit shorter to fit in stretching & strength training, even take a day off each week if necessary. Finally, 7 hours a week is plenty of time to make significant improvement if you spend it wisely.
