Have you seen this thread?
http://www.tenerife-training.net/Foro-C ... .php?t=182
You can get someone to guide you. Either lavatrax in the South, MTB active or myself in the North.
Of course if you can speak Spanish and you have your own bikes, there's nothing stopping you from going down to either Puerto de la Cruz or Las Americas bus station on the weekend at 8:00am and asking the locals to show you the routes. Although keeping up with those crazy bikers is another matter.