The longest day provides the ultimate climax to the walk, and it is likely to take you all day as well. In addition, there is the detour up to the big boggy fell of Cheviot (815) which will add slightly to the distance. Be prepared, it is a hard, long walk and pretty lonely. You must be ready to navigate. 'The Way' climbs over Moorland, with an alternative route to take in Chew Green Roman Camp. Eventually you drop down across the Cheviot Fells to alight at Kirk Yetholm, an old borders market village in the middle of nowhere in particular, but just happens to be the end of the Pennine Way. 
Most walkers choose to split this into 2 stages. Byrness to Kirk Yetholm is charged as a 2-day stage.