I have on occasion spoken to Alastair Wood (1960's marathon runner and London to Brighton runner), and he suggested a session he found very beneficial to his speed endurance.
He would jog warm up and then run at marathon pace for 26 mins, 1/2 marathon pace for 13 mins, 6 mile pace for 6 mins, 3 mile pace for 3 mins and then try to finish off with 1 min or more at mile pace. He obviously played around with the length of the session, maybe running 40 mins at marathon pace or adding 10 mile pace but he would increase and decrease the rest of the session pro rata.
I use this session especially when training for longer runs etc but obviously you could tailor it to suit almost any distance training. You may use something similar at the momnent but I find this method quite easy to do (mentally not physically) because you are splitting it up into manageable chunks and it is great for those sprinting for the line Chariots of Fire style finishes.
Hope this is of some use to someone.