Duoro starts in Libson if you take the precruise extension, and ends in Porto, or visa versa. In Libson and Porto the bus rides are not long. The ship docks where you can walk over the bridge to Porto.
Of course you can opt for optional trips to Sintra and Cascais from Libson, but it is not mandatory. The bus ride from Libson to Porto maybe two hours but you stop for lunch and visit the University v
The one long bus ride on Uniworld was to Salamanca, Spain. It was worth it. The other excursions involve going to wineries from the ship. Some are up in the mountains.