In reply to The New NickB: Earlier in the year I made big improvements ( from a rather low starting point) based largely on a few months of 40mpw average of which rarely more than 10mpw were run at faster than HM + 90s pace.
I picked this advice up from another forum I use, and the arguments for a strategy like this working were several fold.
Firstly running easy pace allows you to increase the volume without risking injury.
Secondly, it means when you do speed work (intervals/tempo) you can go out really hard on it as your legs/body isn't tired from the previous sessions. You can also run the day after a very hard session, in a way you couldn't if you tried to run it at anything other than easy, and it can help with recovery (a critical part of making training work).
Thirdly, and possibly most significantly for endurance training it is supposed to be more effective for building an aerobic base. If you run very hard you build up the power, turnover etc. associated with quick running. If you run slowly you stimulate the body to become more effective at using fat as its fuel source. If you run just sub tempo, you do neither - it doesn't work it hard enough to make marked power/pace improvements, but still relies on using the glycogen (I think) stored in your muscles, and whilst it may have no ill effect on relatively short runs, it won't be making the improvements it potentially could.
Or at least this is my understanding of the reasoning for lots of training schedules advocating easy runs (including lsr ) very easily.