It's a brisk 15 minute walk from the exit of either Terminal 1 (AA) or Terminal 2 (AS) to BART, plus the distance from your gate to the exit. There is a signed path the whole way. Airtrain is faster, though I usually walk for the exercise. From T2 there is a shortcut through the lower level garage that can cut the trip in half but there are no signs and you're likely to get lost on your first try.