Hello, I hope this helps. I've ridden the canal towpath twice this year, once on a road bike whilst heading down towards Nantes and points south, and once on a hardtail semi-tourer with panniers and a tent.
The surface from Carhaix southwards is a little bumpy as far as Rostrenen, but after that good until Mon Repos abbey. You get a choice there, as you cannot cycle past Lake Guerdelan - either join VV6 for a couple of miles (which means crossing the main road twice) or stay on the towpath for 1km (small unmarked path that obviously follows the canal on the N side - left off the towpath (or right into the cafe!) at Mon Repos, over the bridge, and the path is on the R). You soon have to leave the towpath on the L and head up a steep zig zag. When you get to the top keep straight on - the VV6 joins from the L - and it's then all downhill past Caurel until you get to Mur de Bretagne. The surface is small stones, but decent riding. Be careful when you get to M de B - watch out for the old station on your L as you arrive and turn sharp R, almost back on yourself, and this will take you down to the towpath again. Bear L for Pontive.
This section is often old tarmac, and it's bumpy whatever you are riding - grit your teeth! As you get into Pontivy the towpath swings L and heads for Rohan - it's a bit rough and lumpy in places but quite ridable. Look out for the flight of 12 locks - downwards! - enjoy the ride! As you approach Rohan the path improves considerably, and from here until you get to Josellin it's about as good as it gets. After Josselin you will head for Malestroit and the riding is still good, but after that! - it gets a bit more difficult as the gravel gets a bit bigger. Still ridable on a road bike, but slow down a bit! Watch out for crossways drainage chanels across the towpath, or you will be in orbit! It's like this for a lot of the way to Redon, after which it gets a bit easier until you go past Pont Mini, where it gets a bit rough again. Stick with it - not far to go now - slow down and enjoy the solitude!
For a book on the canal try
http://www.bicyclebooks.co.uk where they have a book about walking the canal - decent maps, just reverse the direction. If you do get fed up with the towpath there are many 'detours' where you can hop off and on again - and be prepared to be a bit frustrated at times as you find yourself heading in completely the opposite direction to he one that you want to go in - it does bend and twist about a lot. By following the canal towpath from Carhaix to Nantes you will do an extra 100km over the direct road milage!
Accommodation at Carhaix, Rostrenen, Gouarec, M de B, Pontivy, Redon, Josselin, Malestroit, Redon, Pont Mini, Blain, Nantes. Really make sure that you have food and water with you - it gets a bit isolated at times, and there are few facilites along the way, apart from the larger towns. Supermarkets at Carhaix, Rostrenen, Pontivy, Rohan (Proxi right next to the bridge), Josselin (although it's a long way out), Malestroit, Redon, Blain, Nantes. Camping everywhere - Carhaix, Rostrenen, Gouarec, M de B, Pontivy, Rohan (good site on the RHS as you enter), Le Roc st Andre (just past the bridge), Malestroit, Redon, Pont Mini, Blain, Nantes - or fly camp wherever you like!
If you are heading for Carhaix from the north coast the VV7 from Morlaix to Carhaix is also quite a decent ride, whatever you are on. (
http://www.randobreizh.org)(
http://www.af3v.org). A bit hillier than the towpath (Monts d'Arree 235m) but well graded as it's an old railway line.
How's that? - there are some other links to accomodation on the bicyclebooks.co.uk site, or PM me if you need any further help. I'm off in 10 days to ride from Roscoff down to Limoges, and the first part of the ride will be - on VV6 and the towpath!
kind regards, Braz.