Best way to do it is boat to CR, monorail to Poly. The boat runs pretty frequently so there shouldn't be a problem. We left like an hour before our ressie at Ohana, and we ended up having to kill about 40 minutes. Everything was in our favor for transport (boat pulling up to dock as we got there, monorail at the CR as we got there etc). I'd leave at least a 40 minute window. If you're really early, the Poly is beautiful to explore. Generally the WL shares busses either with GF or Fort Wilderness, so it's not a good idea to try to hop a bus to the Poly.