There is a large park and lake about a mile north of Ghent if you ever visit on a nice hot day.
It's all man made inc the beach around it. You can also run along the sandy footpaths that wind through the trees in this park. They have also set up a number of exercise apparatus along these paths eg monkey bars, balance beams, chin-up, sit-up, press-up beams etc.
Apparently my friends told me these were introduced by the local council as there was a fear that local residents were becoming overweight!
Oostend has a massive pier which would be spot on and I'm sure Brussels must have plenty of large parks away from the city centre as it's such a big city.