Looking for some half-marathon training advice

I am looking for some half-marathon training advice.  I know there are loads of threads like this, so apologies if this is covered elsewhere.

In summary, I ran Bideford Half marathon on 4th March and got a PB (1:42:02).  Bideford is a fast course.  My main goal is a marathon at the start of May for which I am following the Shades plan, but I am looking to a June Half (Torbay, which isn't blindingly quick) on a course I know well to see if I can get under 1:40.  The reason I am targeting this is because in June 2012 I struggled around in 2hr+, so if I was able to do it I think knocking 20mins off in one year would be something of a milestone.

I ran the Bideford Half with a 4-minute negative split, but the course profile is suited to that whereas Torbay isn't.  Even so, holding back in the first half and being strong in the second felt much better than blowing up in mile 10, which has happened regularly to me (it actually happened 3-weeks previously to Bideford when I ran Exeter, which is flat).

For info, I'm male, 12st 10, 6ft 3, 37 years old.  I run 4 times a week with 1 LSR at 10:00--10:30 min/mile, 1 hard club hill/speed session, and 2 other slowish runs (maybe 8:30-9:30 min/mile).  I currently run between 32 and 42 miles per week depending on what the marathon plan says, with the main emphasis being on the LSR ran slowly.

So, with the above in mind, is there any advice on what I should do after the May marathon in terms of training?  Knocking 2mins+ off seems doable - do you agree?  More speed work?  Less mileage on the LSR?  Introduce a 5th session (to be honest, I'm not sure if I can commit to this)?