I've probably hit my current endurance limits for the moment, with my Maffetone-method long run finishing at a touch short of 18km. I'm slightly disappointed that I couldn't go on with it, however, it rounds out a 60km week, much of that distance completed at an honest work rate, so nothing to be disappointed about, really.
I'm now at the point of having to consider intra-running nutrition a little more seriously. The long run was again done on zero fuel, that is, before breakfast, with nothing but water throughout. Whilst I am pretty sure I'm now burning a high percentage of fat on these runs, I should perhaps be adding some small amount of carbs, most likely in the form of glucose, to my drinking water. This small percentage of carbs is apparently required to trigger and maintain the fat-burning process. More experimentation is required.
But hey, 60km is 60km! Now to make it a ripper month.