It’s a worth it excursion to head 20 km east of Utrecht to enjoy a run in Heuvelrug National Park. The 6,000 hectare park features a good variety of landscapes, including heathlands, sand dunes, forests, grass lands and floodplains. There are many km of trails and paths in the park — too overwhelming to recommend a specific route! In fact, there are 56 designated ‘walking routes’ in the park, ranging from 2 to 21 km, many of which are great for running/trail running. Some are more circular ‘walks’, while others are more designated for hiking. The hiking routes can be designated for trail running and might have narrower trails, or shifting surfaces such as soft sand. There are even hills in this park!
This site has good information (use translate!) on the different walking route options, which you might choose based on desired length or type of terrain/geography.