If you will be in Northern Peloponnesos, there is the old national highway, with absolutely fantastic vistas of mountains, sea, sunsets, and incredible nature. You will encounter water the color of sapphire, rolling hills nestled with olive, lemon, orange, and eucalyptus trees perfuming the air, as you gaze down at the picturesque alcoves of hidden pebbled beaches, stately churches, and views of ancient volcanic mountainsides dotted willy nilly with wild flowers. You will pass through small fishing villages with tavernas serving freshly caught seafood, beachfront/view restaurants serving a wide range of delightful, fresh, and on the whole healthy country fare, and will come across guesthouses and sea-side bungalows for rent, low cost hotels, and even campgrounds dotted periodically along the route.

All this said, here's my big, no, HUGE warning:

I don't know where you're from or what type of driving habits you are used to (I'm from the New York City Metropolitan Area, and have experience commuting to work on bike), but I absolutely, positively guarantee that you will need time to adjust to the driving style of Greek motorists in order to gauge your own competence in sharing a road with cars in Greece, unless you are like me, and are Greek, and have spent considerable time in Greece--and I would still hesitate in deciding to travel more than 20 miles in any direction from my father's native city.

I was just there a few weeks ago (Patras, in north-west Peloponnesos), and I can report that I did see a fair amount of bicycle tourists, mostly couples, meandering through the seaside towns, and mostly avoiding country and mountain roads (although I did spot two sets of bicycle tourists on an especially windy and narrow stretch of mountain road on the other side of the Gulf of Patras.)

Here are my main concerns for anyone planning a cycle trip in Greece. First, the roads in Greece on the whole are not maintained as well as many other countries in Europe that tourists frequent. I have heard about people traversing northern Greece, or sections of northern Greece, but I am not too familiar with the area, so I'll leave that alone for the moment. Many of the roads that you are likely to encounter in Peloponnesos will be windy and narrow, and have not been engineered for high commuter traffic or heavy commercial traffic. That said, these roads tend to be packed with 18 wheelers/lorries overtaking one another, while cars straddle the shoulders in the opposite direction. In fact, the shoulder is most often used as a sort of "third" or in sometimes "second" lane in either direction. There is also a different sense of distance between cars and the shoulder/edge of the road in general. Greeks tend to drive very close to one another, so they might not see you on the side of the road, and you might not see them following behind the car in front of them.

If I were you and your wife, (and I will assume do not have intensions on crossing over into neighboring countries) I would find a nice little island (you have about 10,000 to chose from) and plan a seaside cycle tour with as few cars/tourists as possible. Check out Ithaca, Lefkada, and possibly Corfu for starters.

Good luck, send postcards!