I can also recommend EasyRide (
www.easyride.reth.gr ) we have hired bikes from them on our last 5 visits. They hire both MTB and Hybred bikes and i would recommened a MTB for the area around Plakias, especially if you hire one with a remote front suspention lockout.
Whilst most routes involved some climbing following the irrigation canals is a very relaxing ride after the initial climb out of plakias
The climb up the gorge is quite well graded after leaving Mirthos, but usually has a headwind although in a strange way this is good as it helps keep you cool. A good route is to climb the kotsifos george then it is downhill to Angouseliana where you can either contine to the main rethymon road turning right and right again to Koxare or cut across county on the back roads dirct to there. Fom Koxare it is largly downhill to Plakias through the Kouftaliotis george with just a sharp climb into Asomatos.
I suggest you get a copy of Lance Chiltons walkers map (avoiding if you can the blatent copy sold under the Hapimag name)
Finally i would recommend avoiding the direct route to Selia from behind Ostraco and instead using the road that leaves the coast by the new harbour, as it is very steep as is the concrete road direct to Merithos