Spring (April to May) is the best time for hiking. The countryside is green and fresh from the winter rains, and carpeted with wildflowers. Autumn (September to October) is also a good time.
Several companies run organized treks. The biggest is Trekking Hellas which offers a variety of hikes ranging from a four-hour stroll through the Lousios Gorge in the Peloponnese to a four-day hike around Mt Olympus.
If you’re venturing off the beaten track, a good map is essential. Most tourist maps are inadequate; the best hiking maps for the islands are produced by Anavasi and Terrain, both Greece-based companies. Be realistic about your abilities and always inform your guesthouse or local hiking association of your planned route before setting out.