Gary it's a very nasty virus - short and sharp but it can leave you very weak, particularly as you can't eat. I had it last Christmas and didn't run for a week.
With regard to your training - if it helps, I was injured with 3 weeks to go before a HM in October. I didn't run at all for 2 weeks then only did one 5k before the race. I still managed to hit my target time, although it possibly took a little more effort. The main point was that I was fully recovered.
What I'm saying, in a very long winded way, is that in a few days you'll probably be feeling significantly better. Rest, drink lots of fluids, eat normally and when you do start running you'll probably be fine. You'll have lost very little fitness, whatever the voices in your head tell you to the contrary!
Edited: 19/11/2012 at 11:31