PAz - it all depends what you want to achieve at VLM. If you are looking for a fantastic time, then you will need to be doing 5 or 6 sessions a week. But if you want to get round in a reasonable time and enjoy the experience, what you are doing sounds fine, If you can build up a good base firness over the next few months, get the race experience during your halfs, then start expanding your long runs in the New Year, you'll be fine.
I'd get a couple of programmes and see what the key elements are and fit those around your job.
Chris - I think the same would apply to you. And I would expect you to drop you time by 1 - 1.30 per mile over the new few months - I know I did when I first started.
Paz - the other option is - I usually marshall those 2 halfs so you could also bribe me 