Are you sitting down? Then we shall begin ...
Start
http://routebuilder.org/o8rDown to Shawbirch island and back to Prees island twice. (giving 95.5 miles) Then ...
Quina Brook circuit
http://routebuilder.org/o8t6 times. (giving 171.2 miles) Then ...
Prees island down to Shawbirch island and back 4 times. (giving 368.6 miles)Then...
Quina Brook circuit 6 times (giving 444.3 miles)
Then continue A41/A49 to Grindley Brook island, left on A41 to Broxton Island continue A41 (giving 461.1 miles)
Then...
Finishing circuit
http://routebuilder.org/o8y which is 13.572 miles long, giving 474 miles on completing the 1st lap, plus whatever else you can do.
Thank goodness for the kind efforts of the marshalls and sign putter-outers, without whom it would be quite challenging to be sure of where to go, and when.