Well I discovered yesterday whilst out of the bike, that if I leave my gym, cycle to Henley, do one of lap of the Challenge course then cycle home it's exactly 56 miles! And I know if I do the local HM course from my house (missing a little bit in the middle) it's 13.1 miles. So after a quick word with the gym, that's my HIM sorted. 1900m in the pool, T1 is the manager's office, loop of Henley, T2 my place, then the local HM course! Sorted! 
Now, feed stations.....mmmmm.....??