5 Weeks before my 10k, need to get some speed.

I've got a 10k coming up at the end of May, and need get some speed to try to get close to my PB of 40.14.   In the last month I've run a half marathon where I ran a PB and a last minute 10 mile race entry where I came in 3 minutes off my PB last week.  I've been doing 40 miles a week regularly for 18 month but haven't done any speedwork for 5 weeks, so I'm looking for the workouts that will make the most difference quickly. Any ideas?