Britannia is big, and walking it is half the game. These save the other half.
Moongates
Eight city moongates: Minoc, Britain, Yew, Trinsic, Jhelom, Skara Brae and Magincia take you; Moonglow's is dark. Step on a gate and choose.
Horses
Stablemasters sell horses, and Minoc's stable yard has three free ones. A ridden horse goes through a moongate with you. Dungeons and logging out set you down on foot.
Ferries
Every port has a ferryman. Say "passage" at the quay to hear where he sails, then "passage to Jhelom" to pay and sail. The fare starts at 8 gold and grows with the distance.
Recall and runes
Mark a rune with the Mark spell, then cast Recall on it to go back. Mira the Provisioner in Britain sells runes and runebooks, and recall scrolls work for those who are not mages.