and me IMO the best way to improve your speed is to build a strong base, with a strong base speed comes naturally. I know i keep saying it but when I started training i could only run/walk at around 12 min miles on the flat to keep my HR at 142 bpm when I am in shape I can run at 7.45 min miles at the same rate, it therefore stands to reason that when you want to push you have plenty in reserve. The key for me is to increase your pace at low intensity and the only way to do that is to run lots of slow miles.
depending on how long you have been running? I would think that should get you sub 1:40 My only thought on 35 miles a week you may be doing to many speed runs. I would drop one in place of more easy miles. most of my runs are at 142 bpm, that equates to 7.30-11.30 min miles depending on the terrain. 6 weeks from a race i introduce 1 speed run a week of around 4-6 miles.