According to the scene-by-scene times given in the Reunion Files, Cloud woke up at home around 5:30 p.m., talked to the Turks, talked to Tifa after she woke up, and made the journey from Edge to the Ancients' city by 9:30 p.m. He then fought the silver-haired trio, met up with Marlene in the woods hours later at 3:30 in the morning, took her back to Edge, and met up with his friends to battle Bahamut by 2:00 in the afternoon.
Considering Cloud made the trip from Edge to the City of the Ancients in what must have been far less than four hours (time for talking to the Turks, waiting for Tifa to awaken, and then talking to Tifa), a trip from Nibelheim to Midgar in two or three days' time seems infinitely more sober by comparison.
As for Zack riding that bike over mountains and oceans, Force made a great point a few posts ago indicating why it's more likely he only ever rode that bike in LO on the Midgar continent. He and Cloud had escaped some 72 hours prior, but they weren't using that bike the whole way. They were apparently footing it most of the way.