This is my “neck of the woods” now since moving from Brighton to Arundel a few weeks ago.
Whatever sort of riding you’re into there should be something nearby to whet your appetite.
If your the sort of rider who sports ergonomic grips and wears lycra then you’ve got the whole of the South Downs and the SDW to explore! Quest in Worthing to Thurs rides every other week. I’m sure they’d welcome you along.
If you get your thrills from squeezing between tight gaps between trees and value a good bit of singletrack then there is Stanmer Park in Brighton and Houghton Forest(Whiteways). Brighton MTB do rides Thur eve(and Tue if you’re resonably fit) in Brighton and worth the trip. I think D&D cycles in Barnham do Wed eve rides in Houghton Forest.
If you use words like rad, roost and railed regularly and like to spend time either in the air or on the brakes then there are some excellent and very well maintained push up and ride down trails at Steyning.
The road riding is nice around here, but I wouldn’t like to be out in the evenings mid-week on the roads.