The info about the park says this is the biggest public space in Essex, and it is huge.  Mountain biking is the big deal here, it was the site for the Olympics in 2012 and since then has become the top spot for UK mountain bikes. There's still more than enough space for dog walkers, and the paths are well maintained and accessible for wheelchairs and mobiity scooters. The park website suggests two routes, and there's scope for making your own route and having a thorough eplore if you live nearby.  We liked the walk to the castle and back

Dog Walk  Round trip to Hadleigh Castle: From Chapel Lane car park, walk downhill until you reach the marsh, turn left towards the castle and follow the path until you reach the farm track, rising steeply to the castle. To return to Chapel Lane car park, head out of the castle towards the Salvation Army tea rooms and follow the road with the white houses - Sea View Terrace - to a farm track which takes you to the park's entrance. It takes about an hour.