How about some fast flat miles (stating slowly though).. This will be a good prep ride for Paris Roubaix. I'm going to aim for 30 laps of Longchamps in total starting fairly sedately, just spinning legs, for the first 10 laps and then building pace slightly after that.
A good place to start if you are worried about your fitness as you can peel off and call it a day as soon as you get tired or bored.
The full ride will be around 110kms and I might even stay on to ride 140kms (depending on the weather) if anyone else is going that Strava challenge this weekend. http://app.strava.com/challenges/quarq-power-trip