“Make haste slowly.”
Or, if we forsake brevity for clarity: get it done quickly, but not so urgently that thoroughness is sacrificed for speed.
Once committed to action, it is desirable to move swiftly, decisively; but to do so without having completed one's due diligence1 is foolhardy at best, and dangerous at worst.
The hard part in all of this, of course, is know when the diligence is done and the planning can begin. Look too hard for the perils and pitfalls, and you'll never get the thing underway. Don't look hard enough, and you might find yourself starting from scratch—or abandoning the project completely. ↩