I knocked off a flat HM at Dorney Lakes earlier this year in 1.41 and did a very hilly South Downs HM (3,000 ft of climb) a few weekends ago in 1.50 - winning my age category (over 50).
My very limited experience is that a HM is as much a mental challenge as anything. If I reach 6 miles and start thinking "Still not even half way yet" then I know I'm in trouble. For what it's worth I ran a few very slow but very long training runs in the months before each HM. I mean 14 to 15 miles in well over two hours.
I can't testify to the physcial benefits, although I'm sure there were some, but mentally I found it much easier to keep going. During the races I kept thinking, this will be over far quicker than my last training run. So I would say running over the distance in training does definitely help - although as I said I am far from an expert and this is simply my own experience.
Edited: 19/06/2012 at 16:27