What you need is entirely dependent of the type of walk you do and at what time of the year and beyond the scope of a short article (I will probably go into these details at a later time) so for now I have only one piece of advice: bring good shoes and preferably walking boots. Almost everybody walking in Crete for the first time (including very experienced walkers) remarks on how rough the ground is. The paths are also not as clearly marked and defined as in many other countries and most people find the walking quite a bit tougher than they expected. This has nothing to do with distances. steepness etc...but is related to the type of terrain.
The huge attraction of Crete is its variety. With its 8500 km² Crete has 4 main climate zones (from alpine to southern Mediterranean), yearly rainfall that ranges between 200 and 3000mm depending on where you are, there is something for everyone.
Crete has over 1000km of coastline, four mountain ranges with over 40 peaks higher than 2000m. If has three times as many native plant species as the whole of Great Britain. Gorges are too numerous to list but the district of Chania alone has about 50 of them besides the famous Samaria gorge. Add to this the fact that half of the population of Crete (about 600.000) resides in 4 towns on the North coast, leaving a population density of 30 inhabitants per km² in the rest of the country and you have all the ingredients for a walking paradise.
As you can imagine given such variety there are good walks everywhere and the rest is down to personal choice and maybe proximity to where you are staying. If you need inspirations check my list of books about walking in Crete.
Given the variety of landscape and climate the walking season is pretty long. Here is what you will find:
Finally, if you are thinking of walking in Crete, take a look at Walking safely in Crete. It takes 5 minutes to read and could save you lots of trouble.