Hmm... how much of a slow down causes the effect do you think and over what timescale?

At the peak of training I was running about 15km further a week than I would normally, and I'm not scaling right back to that since I'm running a half at the end of May (though I have run rather less than that this week), but I'd rather not spend a lot of weeks doing runs of over 18miles (I reckon the 18 mile point is more or less where it stops being fun and starts being a grind).