You've got 4 of your 5 long runs in the bank, so I wouldn't worry too much. Psychologically its nice to complete that last long run, but physically you'll have made most of the adaptations on your previous runs. Also most people don't complete 100% of their plans due to illness or other commitments, so your not alone.
Best thing you can do is make sure you have recovered from the flu, and then continue with your plan. I'm assuming it will have two or three weeks of tapering, so just continue with the reduced miles. Your more likely to hinder your race if you try and "squeeze" that long run into the last couple of weeks.
Also remember that it takes two weeks for your body to benefit from a particular training session, so the last couple of weeks are about maintaining your fitness rather and making sure your rested rather than trying to boost your potential performance levels.