Go to the search page, and search on your postcode, this will produce a list of 500 caches, so go to the last page in the search and look at the distance for the last cache on that page (number 500).
Hmm! I tried that but ended up with over 27000 caches, and what's more - they were all in Germany. Don't think I'll bother again.
I could only limit my search to 500 after signing up for Premium Membership.
11.5 miles for the nearest 500 in north Herts, I'm pleasantly surprised that I can do so many on my bike.