i just got hit hard by the flu in the past 24 hours. i actually went for a ride this morning and only managed 7 miles at a slow pace and i couldn't ever get warmed up. i was shivering so i just went home.
in the past, i've always waited until i felt good and then tried riding on a trainer for a little while. basically just to move the legs a little and see how it goes. that way i can stop without all the hassle of changing clothes to go outside, etc.
the flu is a little deceptive sometimes, in that you may feel good for periods of time and then get worse a few hours later. so you gotta be careful. for me, it takes a good solid 1-2 weeks after it's cleared to get my fitness back. if it's a bad one, it may take a couple weeks longer.