Once you get out out of Bru-town onto the Ring Rd ( somewhat akin to M25 , but : very , very much-better-moving !!! ) you have the choice of 2 main Autoroutes :
a ) westerly to Ghent , then turn off southerly to Courtrai/Kortrijk and from there onto Lille - some 100 km in all , I'm told ;
b ) southwesterly towards Halle , Enghien and to Tournai , then : Lille , some 112 km in all , but more pictoresque and always less traffic (the route less travelled ! ) .
As for sensible speeds of travel : posted limit is 120 km/h , at 130 km/h most traffic overtakes you and leaves you behind quickly ; in my 7 years of cruising these M-ways , I have never been even inconvenienced by traffic cops , fines or similar nuissances , for travelling at around 140 , but that's : in Belgium !
( Wouldn't dare do any such thing/s in next-door krazy-speed-cop/camera-Netherlands , nor in our ever-so-repressive U.K..!!! )