This is easily done in half a day (transport from Antigua, included). I have been twice, both times with Old Town Outfitters and I cannot recommend them enough - their guides are professional, conversational, interesting and encouraging. What's not to like about that?
The hike got me huffing and puffing, and most people will be able to make it up. One of the funny things about the first half of the hike is that locals on horses will literally follow you up that first (and steepest part) waiting for you to "cave" and buy the horse ride! Hearing the sound of horse hooves and the occasional snort behind me made me feel that I was being followed by a HEARSE, waiting for their chance to haul my cold, dead body off the mountain...for a fee!
At the top of the lava field, David and other very cool artisans have created the Lava Store that sells lava-based jewelry as a social enterprise. He's great to talk to.
Bring a ham and cheese sandwich to cook-up in some of the "lava ovens" that exist there.
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