Venice Train station to La Loggia della Luna

Check Google Maps. Possibly walking is fastest from the train station. If your flight isn't crack of dawn, I'd look into taking an airport bus from Piazzale Roma, which isn't far from the train station, so unless you have huge bags that might be walkable too.

You are basically right across the canal from the station, so anything other than walking would be a waste of time. Walk out the front of the station, cross the bridge you'll see to your left, and then another left. Print off a map from Google Maps and you're all set.

As for the airport, it depends how much you want to walk. If it were me, I would simply walk back past the station to Piazzale Roma and catch the ATVO bus to the airport. For slightly less walking, take vaporetto line 1 to Piazzale Roma.

All of these details are on Google Maps; I encourage you to have a look there.