[puerilesenile]

Certainly not an easy fold. SB could have been floating you on the flop knowing that it is unlikely you have an ace. I would reluctantly fold because SB has a big stack and there is no way you can call a big bet on the river. I also prefer a bigger raise on the flop. Makes it seem less like a value raise and more as if you're telling the SB that you know he doesn't have an ace. Since he wouldn't expect you to have an ace this is more likely to induce a 3-bet bluff and you can shove.