My firs mara was VLM, it's fantastic, you'll love it. Forget what your HM time is, it's a completely different beast. There's no reason why you shouldn't make it your first.
However, you will not run a 2.39 off a 1.20. That's an odd race time predictor you're using. Even say a 2.47 will be tough unless you suddenly turn in a sub 1.17 HM.
Key is to set a time (probably 2.55) and then stick to it, period. It's so easy to get carried away and go for a faster time as the first 13.1 miles will feel like you're walking.
I was down as pen 3 but managed to sneak into pen 1 and had a clear race, but they've really tightened up on this.
Edited: 15/09/2012 at 00:21