Mark, are you severely hampered by time or something?
You're basically doing 3 quality sessions and a long run, which in your situation becomes semi quality.
It's the equivalent of trying to put the decorations on your house without building any foundations!
Put simply you don't have the base to back up the sharp stuff, and doing sessions like 5k flat out, combined then with 5k extra of speedwork, can only see you plateau!
I'd have a massive rethink, and work to a 3month or so plan, to build a base. Get the mileage progressively up from it's current 25 or so miles a week, up to 35-40, safely doing this by dropping the fast work.
Once you can handle that mileage, then add a speed session back in, making sure it's calculated to your current fitness.
I'd say that'd comfortably get you sub 40.
Not sure you'd ever really need to do more than 2 quality sessions a week, otherwise the recovery starts to become a problem, especially if you're doing big miles.