Thought I'd throw this one open to the floor for opinion, comment whatever.
Basically, I'm mainly a 5K/10K runner (comfortably sub 18 for 5K, about mid-37s for 10K), but I've responded to a bit of forumite pressure and put my name down for the Helsby half-marathon in January - as a one-off. This despite the fact that until 3-4 weeks ago I'd not run more than about 7 miles since the 1980s. What finally clinched it for me was the thought that the extra mileage would if nothing else give me a decent aerobic base for trying to improve my performance at my favoured distances.
I've tried to come up with a plausible target time to go for - 84 minutes is what I've guessed at. This would be about 6:24 pace - don't know if it's reasonable to expect that from the above performances at 5K & 10K.
Current training regime is as follows:
Sun: long run (currently up to 10 miles, will lengthen further)
Tue: ~7 miles (inc. warm-up/warm down) - quality session with Wirral AC runners (could be speed session, hill reps, tempo....) - v hard - they're all 37mins pace or faster over 10K.
Wed: 5 miles recovery
Thu: rest or 3-4 miles short acceleration run (alternate weeks)
Fri: longish run (~Sunday length minus 2 miles), higher effort level than Sun
Sat: 3 miles or less, brisk.
As you can see, there are only 2 outings that are really relevant to the 1/2M - on Friday and Sunday. Is that enough? I don't want to drop the Tuesday session - but its intensity more or less determines what I have to do on Mondays & Wednesdays.