It’s probably about a 90 minute walk from the locks to the hill fort and/or white horse. I took a 360 pano of the lock during the GADC Birthday Comp. You can see the hill in the background.
Google Maps takes you the long way around though, so for anyone planning a trip, below is what most folk use to get there. It is only then a 10 minute walk to the hill fort.
I also added more detailed directions by car and foot to my Drone Scene post.