Geograph - photograph every grid square

The Grand Union is a 137 mile (220km), 166 lock, 2 tunnel canal running between Brentford in London and Birmingham. It is the longest canal in Britain, and has 11 main arms and junctions. It takes over a week to navigate.

The Leicester arm branches off at Norton Junction and runs 66 miles long through 59 locks (including 2 staircase locks at Foxton) and 2 tunnels to Leicester where it continues into the River Soar.

Read more at Wikipedia Link

A milestone is one of a series of numbered markers placed along a road, canal, railway or sometimes paths at intervals of one mile or occasionally, parts of a mile. They are typically located at the side of the route. They are alternatively known as mileposts or mile posts (abbreviated MP on maps). Mileage is the distance along the route from a fixed starting point.
Milestones are constructed to provide reference points along the route. This can be used to reassure travellers that the correct route is being followed, and to indicate either distance travelled or the remaining distance to a destination.
Wikipedia: Link