The alternative southern end of Lon Las Cymru is exceptionally hilly but worthwhile. The climbing starts before you've left Chepstow, and it's a never-ending succession on very quiet roads until you drop into the Usk valley. Usk itself has good pubs to replenish your energy. It's slightly less hilly from here to Abergavenny, where you can choose to go via the thriving town centre or to cut the corner via quiet roads to the north. After this comes the big one: the long ascent of the Vale of Ewyas, past Llanthony Priory, up to the Gospel Pass - Wales's highest road pass at 549m. It starts steady but you'll be pushing 15% by the end. The view from the top makes it all worthwhile, and from here there's a precipitous descent to much-loved Hay-on-Wye, its cafes and bookshops. From nearby Glasbury, you can follow NCN 8 (Lon Las Cymru) the length of Wales to Holyhead.