If you are back packing in Greece or even if you are not, it's highly recommended that you obtain a wheeled version of your style of preferred luggage or alternatively a fold up cart with wheels for schlepping your belongings to and fro within Greece. From the metro-train to the boat, from the dock to the hotel, etc. The "back pack with wheels combinations" are hard to find and expensive when you do!
Get a sturdy one too as sidewalks are very narrow in Greece away from main boulevards, and will blocked by motor bikes, cars, sign posts, other people, dogs laying in your path and who knows what.
Traveling on foot with luggage you'll often be in the street more than on the sidewalks where there are any.Vast tracts of Greece are either abandoned or under construction. In parts of Athens and on a lot of Greek islands side walks weren't as big a part of the original plan as many westerners are used to today. In fact oft times you'll notice there was no plan which is part of Greece's charm.. but not so charming when you are in transit on foot with luggage!
Rural areas of Greece are in decline and many, many, many people have moved to urban centers particularly to Athens. Much like other migrating species in summer, many Greeks tend to go where the food (livelihood) is, and those areas are the popular tourist destinations. Here you'll find the most development and construction.
Many mountain side Greek villages don't have sidewalks except around the main square. Flag stone or rock paved lanes with steps designed for mules and donkeys are a common sight.
In Athens, away from the historic center, the side walks are the most annoying aspect about coping with the city on foot
I went to Marshall's (a US discount store chain) in Florida before I left the US for Greece and bought a wheeled Perry Ellis suitcase. I'm not much for designer stuff, but it was black and low key and only cost $38. As I was leaving my house to go to the airport I rolled it down the 3 front steps and one of the wheels broke. Talk about luck, me the great white hunter with a travel guide about Greece and everything. stung! Of course it was too late to take it back, the plane was leaving in a couple of hours and where was the receipt anyway?
If I were you I'd do a full test run before I left for my trip. Usually they allow you 70 lb. of weight. If the flight isn't full, sometimes they let you take more. If you are over weight and they catch you they charge extra by the pound (@$15/LB). Your bathroom scale will help you check your baggage weight.