I had the same problem last week. Although I'm not running a distance race anytime soon I still wanted to get back on track for my long run schedule as soon as possible.
I found that starting off slow with a 5K at a much reduced pace was helpful. For me it was more a matter of getting my cardio and muscles reaclimated with running. The first two days back I only did a 5K. The third day I took a break and on day 4 did a 10K at slightly less than my normal pace. Day 5 entailed an 8 mile run. By Saturday I was able to get a good long run in of 15 miles and felt good. Still a little slower than normal but my muscles and cardio were good.
Also, you'll probably want to hydrate yourself a little more than normal prior to your runs. The flu really took a toll on my hydration so I found myself needing more water than normal, even for the lower distance runs.