Leuven is 10 minutes walk to the central square down one road; the station is arguably right on the edge of the centre itself so you're really there when you come out.
Liege is a larger city and it depends what you mean by the centre. If you're coming into Liege Guillemins station, Place St Lambert, arguably the biggest central square, is about half an hour's walk away. But you're certainly in a city centre as soon as you exit the station.
Alternatively there's Liege St Lambert station which is right next to the aforementioned square, but has a more limited selection of services (and is not on the high-speed line).