The Hobokense Polder Nature Preserve, bordering the Scheldt River 5 km southwest of central Antwerp, is a lovely spot for a run. There’s a nice variety among the 9 km of paths: woods, grasslands, a riparian swamp, and a 2 km section along the river. The well-marked paths are divided into several possible walking loops, which you can combine to create your own enjoyable run. The paths are gravel/dirt — it’s fairly gentle trail running.
Hobokense Polder is an easy 15-minute train ride from central Antwerp.