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This 13 mile (21km) surfaced path through the Sussex countryside is mostly traffic free and is a safe route for walkers and cyclists of all ages, forming part of the National Cycle Network – route 21.

The picturesque trail follows the former 'Cuckoo Line' railway track and stretches from Heathfield to Eastbourne Park. It passes through Horam, Hailsham and Polegate.
Link

The Cuckoo Line is a now disused railway line which linked Polegate and Eridge in East Sussex, from 1880 to 1968. The railway line served the following Sussex communities: Polegate, Hailsham, Hellingly, Horam for Waldron, Heathfield, Mayfield, Rotherfield and Eridge. Services continued through Eridge and onward via Groombridge to Tunbridge Wells.
The Hailsham-Eridge section closed in 1965, the Polegate-Hailsham branch surviving until 1968. Eridge-Tunbridge Wells closed in 1985, and this line has been resurrected as the Spa Valley Railway.
The section of the line from Polegate to Heathfield is now National Cycle Network Route 21 and The Cuckoo Trail.
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