At the very least, put up a landing page ASAP to collect contact info from random passers by.

Suppose the ideal lag time between "coming soon" and launch is 3 months -- just for the sake of argument. If it takes you 12 months to build and test the app before launch, which position would you rather be in?

(A) 9 months of passive data-gathering + 3 months of active pre-launch promotions
(B) 9 months of missed opportunities + 3 months of active pre-launch promotions