I wouldn't advise walking. The closest towns are Santa Eulalia and Jesus. Both are a very long walk (over an hour to Santa Eulalia) along a main road with no pavement and Jesus is not worth the effort. Your best bet is to use a taxi (around ten euros to Santa Eulalia), a bus or there is a ferry that goes to Ibiza town. Better still, rent a car for a few days; it's a beautiful island and you won't be able to see the best of it using public transport.