David - Thanks for mentioning the eco-camp - I'll make sure I check that out when I get there. For the record, I've just started work on a five-volume series of guidebooks that are planned to cover all seven of the Canary Islands, but don't rush to the shops for a couple of years yet, and it might well turn out to be five years from now when the last in the series is finally published. In the meantime, be assured that there is a vast amount of hoof-work going into all this!
As for El Hierro...
When I first went there some years ago, I would have said that it would warrant a week's walking, then you'd either run out of places to walk, or you'd end up traversing steep, rocky and disintegrating slopes that wouldn't be enjoyable. While visiting the place a couple of years ago, I was amazed to find spanking brand new signposts popping up everywhere, and I was given a kind of trail map hot off the printing press. The woman at the tourist office told me what day the map would be in the office, and when I turned up, she actually opened the box containing all the copies and gave me the very first one off the pile. Anyway, on my visit during the last month, I spent over three weeks walking the length and breadth of El Hierro, and covered every single one of their waymarked trails. There's a lot of distance there, and for a small place, an incredible variety of terrain. I busted my guts to cover everything and thoroughly enjoyed it. So, give it as long as you're able to spare, and be prepared to put in some distance. If you take a tent, then you can trek to the far western end of the island and back. No-one lives out that end of the island, so you have to be completely self-sufficient. One very popular path, traversing cliffs below La Pena, has notices either end saying it has been closed due to rock-fall. This is true. Rocks have fallen on the path and caused some damage. I walked it anyway and found it was still passable, but be warned that there are still some hefty blocks of rock waiting to come down!
Oh... and at this time of the year... double-check all the ferries and flights to and from El Hierro. There aren't many, but while I was there, they changed the ferry schedule. I turned up at 3pm for a 4.30pm ferry, only to watch it slide gracefully out of the harbour! Apparently, it was the first day of the new schedule. Using the next ferry, three days later, would have caused me to lose my flight home. In a mild panic, I hitch-hiked the 8km or so to the pocket-sized airport and asked for a seat on the next available flight. (My Spanish isn't great, especially in a crisis, and instead of a "seat", I actually asked for a "toilet" on the next flight. To his credit, the ticket clerk didn't roll on the floor laughing, but got me straight onto the next, and only, flight.) Interestingly, the difference between the ferry and the flight was a mere €15, and I got fantastic views of the northern face of El Teide, which are almost impossible to gain except on one of those flights.
So... go for it... and be sure to give it as much time as you can spare.