Lôn Las Menai is part of Lôn Las Cymru, the Welsh National Cycle Route, which is about 400 km (250 mi) long.
Lôn Las Menai is the section which runs for 6.5 km (4.0 mi) along a section of the former Caernarfon to Bangor railway trackbed. From the north of Caernarfon it just runs to the other side of Y Felinheli.
References
Lôn Las Menai Wikipedia(Text) CC BY-SA