Consistent consistency so far this year, Andrew! And that's what reaps dividends! I would try to increase the distance slightly each week, and then have a cut back week every four or five weeks. And don't neglect the core!
DT: I think you may be right when you suspect that the hamstrings may be doing too much work. I am having a similar issue albeit at much lower distances and particularly when going uphill. Physio said improve strength in the glutes, through squats in particular. One day I'll take notice of her and do some! Am doing a little work on hamstrings, Achilles/calf...and stomach though.
DT: many people like to see at least 5 runs over 20 before London! Any races planned?
Today was a pleasant 66 minutes which takes me up to day 898.