https://www.hellocanaryislands.com/trail-running/tenerife/ruta-0-4-0/
"This route is situated in northern Tenerife. It takes you on the long and slow ascent all the way up to Pico de Teide and back down again, along the same route.Starting at Playa del Socorro the trails take off into a challenging and steep slope up to Mirador del Lance. From here the route goes on dirt tracks through green pines and vegetation up to the Mirador del Asomadero. From here you have spectacular views across the Valley of Orotava and you can also see mount Teide. Continuing along tracks and trails where the vegetation is very dense it is easy to get confused and lose your way.
The ascent continues, and from time to time the landscape changes colour, becoming drier and more open up to Riscos de la Fortaleza. Here the landscape is very arid.At Cañadas del Teide the trails lead to Montaña Blanca. Here you arrive at Huevos del Teide, which are huge rock formations that were sculpted during the last volcanic eruption. From Montaña Blanca the route passes a shelter to Pico del Teide.
This route has amazing sea views and open mountain views. Reaching Pico de Teide makes it a sensational run. It is also a route running through two completely different types of terrain - dirt trails and volcanic trails."
From sea (Playa del Socorro) to the top of Pico de Teide (3718m), the highest point in Spain. This route is about 28 km each way, and gains nearly 4000m, hence the name "0-4-0". A "non-competitive" event was held in 2014 to test the feasibility of an actual race, and fast times were set.
NOTE: It appears that most FKT runners have not stayed on the official trail, and in fact have taken short cuts. On-trail travel is required in the National Park. Please follow the rules and regulations of the National Park.