Late look in today, work the curse for most of us trying to get some decent training in. Ran a 9 mile with 5 at HM pace last night, on my treadmill. The snow wasn't too bad in this part of East Anglia but by the time I got home, work again getting in the way!, the pavements were really messy and icy and probably only suitable for some slow stuff, as Literatin found today. 9 miles is hard mentally on a treadmill and having the middle 5 at tempo, consistent tempo, I found tough. I used heart rate rather than speed because I am fairly certain the speed, even after a calibration, is a bit out on my machine, perhaps 0.5mph. Consistent speed does equal consisten heart rate though with only a bit of drift in the final mile or so of the tempo.
HeOw no advice from me but plenty of experienced comments already, might save me asking the same question when I get to my only HM before VLM, at Colchester on the 17th March. Good thing you have actually is time on your side with it being so far in advance of the marathon