Walking, flying, building cities, they're all useful ways of gaining XP. But be warned, chasing XP is a dead-end street, the point of the game isn't to see who has the most XP at the end, I know people who have been playing for years and made huge difference to the game, but are only level 3 or 4, and I know people who are level 16 who really don't do much or enjoy the game. :)