No, you have 3 piles of ten cards each.
Not quite accurate, but yeah, use all the cards. The Age 1 FitS deck will have 15 cards. The Age 1 base/Echoes decks will have 12/13 or fewer depending on the number of players. And the Age 2 through Age 9 base decks will have 9 cards each after the achievements are peeled off of them.
Also if you have enough room you might consider not stacking the decks to simplify drawing and returning cards.
My brother often still plays with a variant of the old mixed base/Echoes deck set-up.