I haven't managed to crack 2:40 yet (2:42 best) but that will be close to my goal for this fall, and I think it's reasonable based on a 73-mid half on a windy day. So I can't tell you how I HAVE trained to get there, but I can tell you how I WILL train to try to get there.
First step is a summer of speed. Meaning track season. My mileage will be a little lighter (say 50-65 typically, ranging between maybe 45 lowest and 75 highest), and most workouts will be shorter, faster stuff with plenty of short races (800-3000). This phase will last until early August.
Then I will transition into half marathon preparation. This period, ending toward the third week in September, will resemble the following period before this Spring's half:
2 Mar 08: 72 miles incl 5k + 3k + 2k w/800/400j @ 3:25/km, 6 x 1k + 2k @ 3:15/km, 9 mile LR
9 Mar 08: 80 miles incl 10 miles @ 5:52, 8 x 800 w/60s rest @ 2:40, no LR
16 Mar 08: 56 miles incl unstructured fartlek (poor weather) and road 10k (34:35 - very cold), no LR
23 Mar 08: 85 miles incl 8 x 400 w/60s rest @ 72-74s (v. windy), 2 x 5k w/800j @ ~ 3:35-4:05/km (extremely windy, cold), no LR
30 Mar 08: 103 miles incl 2 x (8 x 400) w/60s standing & 400m jog rest, 2 x 5k w/800j @ 3:23/km avg, no LR
6 Apr 08: 78 miles incl 8k @ 3:32 + 6 x 800 w/200j @ 2:39, hilly 8k race (27:12), 13 mile LR
13 Apr 08: 98 miles incl 2 x (4 x 400) w/45s/400m jog rest, 8 x 2000 w/200j @ 6:52 avg, no LR
20 Apr 08: 30 miles incl HM @ 1:13:30 (48s PB) – knee injury this week
Then the marathon-specific training will begin. Mileage will go up to whatever I can manage within reason (probably 80-100 mostly), and I'll do two workouts per week. These will all be longer stuff, at up to about half marathon pace (fastest sessions) and down to about marathon pace plus 20-30s/mile. Stuff like 3 x 3 miles, 2 x 6 miles, 10-12 miles continuous, etc etc. With a long run every Sunday. Every second long run will include some up-tempo running at up to about marathon pace. This period should end toward the end of November (got Philly pencilled in for now).
Good luck, and have fun!