PMJ - I equate 1:20 to about 2:50 shape for most folks - not myself as I've never run a 1/2 in a campaign, but I think I'd fit that too. So reckon that you'd need to be well under 1:20 to aim 2:45, so if you could get 1:18/9 by the end of the year and before the VLM campaign starts then you'd be well set. I see 2:45 as a big jump from 2:50 in terms of mileage and training needed. I think you'd need to be 1:17 ish for a 2:45 attempt.
PMJ - understood, it'd be hard to get back to 2:50 shape by Autumn, but with another 6 months until VLM 2014 you have a chance of getting under 2:50 if you can stay healthy. Getting a 1/2 time of 1:18 ish by the end of the year would be a good start.
DVG - similar here but hammys instead of calves. I rarely do any build up races, and only did a very small amount of MP running this time due to a dodgy knee. I think more hard running on the flat would help me (esp in long runs).