Thanks to everyone for all the advice etc. Yesterday I did the walk which ended up 39 miles instead of 36 thanks to a detour for a closed section of canal towpath for engineering works which I had not seen advertised anywhere and then I made a [censored] up of where the actual canal finished before my last mile to the train station. Managed to do it in 12hrs 55mins including one 40 min stop for a pint!.