Apartment is great for 2 people. Although it is not in the center there are many facilities very nearby (nice breakfast bakery/breakfast bars, 3 supermarkets, etc). If you do not want to walk to the center you can take bus number 36 (bus stop around the corner). You can buy a discount bus card at tourist information offices. Regarding the bus: keep an eye on the times for the way back. Buses run regularly until (before) 23.00 hrs. After that time the bus runs (around midnight) one more time!
Susana answers quickly and gave us some good tips. For example she pointed us to the Museum Chillida-Leku. We loved it.