cant really help u with distances too much but if u are planning on getting taxis every where look at taxi prices on ibiza spotlight web site, needless to say i would guess from san anotonio to ibiza town is about 13 miles or so not far though. good luck
It takes about a 20 min walk - or about 2km. Just follow the road outside the hotel to the "McDonalds" roundabout, and then follow Avenida Espana all the way down to Vara Del Rey, and then up the ramparts (sounds like a film with Sid James in!). Or just grab a taxi coz there pretty plentiful and v. cheap.