Luckily the day I went on this tour was cloudy so the heat wasn't too bad. The hike is 16km (8 up and 8 down) and as others have mentioned takes about 5 hours, with stops and time for packed lunch along the way.
The first half of the hike is along dirt tracks which can get a bit slippy if it rains a little - definitely wear decent shoes. This takes you to a viewpoint over the Sierra Negra crater which is breathtaking. The second half of the hike to Volcan Chico is over lava and is like being in another world.
The views along the route are great and the guides stop to tell you about various species of flora, fauna and the volcanos themselves.
A definite highlight of my trip to the Galapagos and a good workout!
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