We did this walk last week starting from Santiago del Teide, just before the petrol station, you will see a sign for Puerto de Santiago with mileage on it, which incidentally varies on each signpost as you go down! Good walk, lovely scenery and a chance to see the changing face of Tenerife from about 50 years ago when there was so much more agriculture and cultivation generally. Quite steep in parts so I found poles a great help, though oh didn't use his and of course proper hiking boots recommended for the loose stuff at the La Caldera end and for the rocks and uneven ground. Took us about 4hrs stopping for photos, lunch etc but most enjoyable and well marked out path. Enjoy !