Here is some added tips to get you on the right track. It's not meant as a full training guide but as a rule of thumb plan to get you moving a little faster no matter what your start point is.
Firstly make sure you are doing two strength based sessions a week either at the gym or body weight exercises, tabata or crossfit are good too and focus on your core (all the core muscles not just your six pack), your hips, your glutes, hamstrings and do some balance exercises for good measure.
Secondly don't neglect your mobility work, stretching or yoga and foam rolling or myofascial release routines. The faster you are going the more important this will be.
Right, now to the running.
Do as per your plan but make sure to include:
1 Tempo Run
Do a 10 to 15 min warm up including some muscle activation exercises.
Then do 6-7km at 10 to 15 seconds slower per kilometre than your goal race pace.
Cool down thoroughly.
1 speed training session
15 to 20 min warm up
800 metres at your best 5km pace time. Then do a recovery jog for 1 to 2 minutes
1.5km at your best 10km pace time then 2 to 3 minutes recovery
3- 4 km at your target half marathon pace or marathon pace then 10 min jog out
Cool down exercises.
1 Long fast run
This can be done during your Longest run or one of the longer ones.
3 to 5km warm up
6-7km at 1 min under your goal race pace
4-5 km at your race pace
continue with slow jog for the rest of the distance on your plan.
Very thorough warm down.
So there you have it in brief. A way to improve your half or full marathon time and get the best out of yourself