You should be able to park in the Txantrea area (or another residential area), with a varying distance to the action in the Casco Viejo (not a bad little walk from Txantrea).
Nevertheless, the hint to park away and take the bus is the best plan. Your car will be fine...and even if you can't find a bus, you can always take a taxi to your car.
That sems a bit odd, doesn't it?