Lucky you didn't do any major engine damage! Strange you needed transmission work. One trick to avoid this in the future is to hang the key on the engine seacock. That will insure you open it. And get in the habit of looking at the exhaust right after startup to make sure water is coming out with the exhaust gas.
i assume you are getting an exhaust smell in the cabin. Could just be oil vaporizing off the engine after the work. Just keep looking.