A friend was on a cruise and wanted to get into Barcelona. We looked up maps of the city. Found the dock where the cruise ship would stop, and a map on which we could figure distances to points of interest - Gaudi buildings were on the list. I remember we decided it was quite walkable for two seniors in good shape.
Obviously, getting from the airport is a different equation, and I've not done BCN, but European public transportation is generally so much better than ours, that I've gotten quite casual about arrivals/departures, figuring the locals have ways and I will follow them and/or ask.
This looks like it might answer lots of your questions:
http://www.barcelona-tourist-guide.c....html#transfer
Romelle