Ridden the roiute in both directions, fairly easy, just keep the sea on your left (not that you can see it most of the time)
However, seriously take note of the wind direction, if it's behind you, then cycle on, if however you have a headwind, then head inland, a long way inland, or better still take the train.
A few years back we had 5 failrly experienced cyclists, even with slipstreaming we were hitting a top speed of 7mph into the wind, three of the team eventually gave up and walked as it was less effort and nearly as fast.
We ended up missing two ferries as it took about two hours to cycle from Dunkirk town to Dunkirk port