Best single workout to run you're best 10k?

Hi, I'm looking to run around 40 minutes for a 10k I've entered in early October(PB is 40.14)..  I normally run around 40 miles a week(5 days) and my long run is around 13-15 miles run around 8.45- 9 min mile pace.  I also try to do a interval/hill/park run a week, a 25-35 minute tempo, and the other runs easy (over 9 minute miling).  

I know I can run under 6 minute mile as have mile PB this year of 5.50 at age of 43, my problem is my pace drops off over 5k and I struggle to maintain even 6.30 once I go over 5k.  Any advice to where I could improve? Thanks