Hi
I'm hoping to do the Marathon this year, having done the Half several times and the Marathon once.
I've always found the Half hard going, even though the Finish is several hundred feet below the Start. I find that those steep little descents and ascents in the first few miles throw my pace control, such as it is, so that I try too hard just when I should be settling down.
I like the event a lot - Half and full - and I think the scenery and the atmosphere are great. When things are tough you can (usually) overtake a walker, which always helps. I know what you mean about miles 12-18, JonnyS, but there's a reason that they call it Salisbury Plain.....