NERunner0353 agree with you about bi-weekly long runs. It will keep one fresh over a long training block.
I also agree that yes long sub-threshold (controlled tempo) runs and medium reps at LT will help one improve over the training block.
One thing to consider is you need to boost running economy over the training block.
Either in the forms of fartlek, track work, and even strides at 5k pace down to 1 mile. If all your doing is work at HM pace or slower you will struggle to break your time goal.
Sessions I like to employ as "B" workouts to compliment the long tempo and LT sessions are things like:
-8 to 12 x 1 min with 1 min jog recovery (start at 10k effort and work your way down to mile effort for the last 2 or 3)
-8 to 12 x 400m with same philosophy, start slow and finish fast
-200m on/off or 30s on/off x 10-16. Again start controlled and do the last few at mile effort.
-10x2min at 10k effort with 2 min jogs
-8 strides after an easy day between workouts
-the best workout to increase your economy for a half marathon is race a 5k several weeks before your goal race.