Crete is a perfect place for a walking tour: the island has a dense network of paths and there are villages with accommodation within walking distance of each other. Most of the time, except in the peak tourist season (the first half of August especially) you can just turn up anywhere and find a room to spend the night. You can organize your own itinerary or do like many less adventurous walkers and book a walking tour with a travel company. Both have their pros and cons but if you come to Crete for the first time, coming with a group may be a better alternative as the tour will have been planned by someone with a good knowledge of the island. Of course, the thrill of doing it on your own will be missing but you may also miss a lot of not-so-nice finding out how things could have been done better.
Most companies offering walking groups in Crete operate from abroad so you will need to look for what is on offer in the country you are coming from because flights to Crete are almost always included in the package. There is plenty to choose from in the United Kingdom, Germany and France. Other countries have much less choice.
Take a look at the Walking with a group page for more on the pros and cons of group holiday walks.