I went down with flu at the end of 2009, also proper bed ridden 'I feel dreadful' stuff, not a bad cold. Developed into a nasty sinus infection. Fun couple of weeks.
Today I went for a run, the first real exercise I've done in just over 2 weeks. Managed about 30 mins (normal for me is 35-40 as I only started in September). It was hard work so I kept to a fairly easy pace, and at the end of my regular route is a small hill which I looked at, convinced myself I could have done it, then stopped and walked.
Feel pretty good now, I know I have some work to do to get back to my previous levels but it doesn't seem too bad. The worst part is thinking about how long it's been since you ran and worrying that you'll never get back to it. You will, and it is worth resting until you're properly well.