Offa's Dyke Footpath (Chepstow-Prestatyn, 280 km)actually situated in the border areas with England and passes thru the latter country also.
Pembrokeshire Coast Path (Amroth-Cardigan, 300 km)
In this section, it is appropriate to attach details of an unofficial long-distance path
Other Long Paths
Wye Valley Path (Rhayader-Chepstow, 170 km)which includes a stretch in England,
Glyndwr's Way (Knighton-Welshpool, 205 km)which loops around Mid-Wales, connecting with the Offa's Dyke Path at both ends.
Severn Way (Plynlimon - Severn Beach near Bristol, 340 km)
- my account
Landsker Borderlands Trail96km
Information from an acquaintance on an unofficial path :- The course of Sarn Helen is definite in only a few places (ref. various Ordnance Survey maps) and in most of these places the track is not a public right of way. Thus, the footpaths that are followed are public ones that are close to the supposed course of Sarn Helen. In several places the possible course is desputed, there being one or two reasonable alternatives.
Sarn Helen: Walking a Roman Road through Wales (ISBN 1-85-284-101-X).