'ON THE WAGON' - refraining from alcohol.
Some say the prisoners on their way to be exectuted at Tyburn Hill on passing pubs could not drink beer as they were 'on the wagon'. It is said that some prisoners were allowed to get off and drink their last pot of beer before being executed. However, some stayed 'on the wagon'.
Perhaps though it may come from the Water Wagons, seen mostly in the USA, therefore 'I'm on the wagon' meant I only drink water.
Whatever the origin it has gone into everyday vocabulary.
My thigh muscles were in spasms and I was wondering how I was ever going to complete the 90 km trek....