This is a beautiful canal ride called the Kreiz Breizh that connects the Loire Atlantic, through the Morbihan to the Finistère. The route design begins in Nantes and ends in Brest, it appears fairly easy to connect to other cycling routes that would end in Douarnenez.
320km of the total 360km journey are along the canal towpath through the countryside, far from the busy roads and beach-side destinations.
Tourism Brittany, has published a full colour map book of the canal routes with all the information you might need to plan your trip.
The SMATAH has developped a guide for the Finistère portion of the canal with lots of maps and additional information http://www.smatah.fr/