I live in Leuven so can help you out to find some routes here. Going to the north is mainly flat, the south is a bit hilly.
In attachment you find already one gpx track going to the North, it has a lot of small and well paved roads. As a dessert, there are two small hills at the end :-). Let me know if it needs to be extended a bit, since it's 52km.

If you would like to, I can also share some routes to the south. How many days will you be in Leuven?