Hey Rollo,
Accept that I write you in English, German would be ok as well but that would take me some more time.
As I live in Belgium I would recommend you to bike from Brussels to Antwerp and to follow the river Schelde till Gent: very beautifull and quite: always bike-roads.
From Gent you can take LF53: a part of Flanders bikeroutes:
http://www.groteroutepaden.be/grlf/lf/overzicht.htm#lfvlIn the neighbourhood of Veurne you go to the area: The Moeren: a part of Belgium and France which is lower than the sealevel:
sometimes very windy but very nice!!!
Then you cross the border and you go to Calais.
So, I hope you like crossing Belgium. Anyway it will be much more relaxed than crossing the mountains.
Do you start in Belgium or do you come from abroad?
Best regards,
Erwin