I did my best half marathon time (and incidentally, best 10k time) while I was building up to FLM2002. I think the regular training running over the distance helped a lot, and definitely imporved the finishing time. It was the difference between running strongly to the end and slowing in the last few miles.
JFTR my best times are 41:01 for the 10k and 1:33:03 for a half, so your 1:40 certainly looks possible, although I suspect more long runs closer to the distance would imporve your chances :o)
Iain