What you need to bring
For the longer tours (2 to 4 hours total time) you will need to carry water and food (energy bars, etc) in a suitable backpack. For the warmer months of the year please bring a running cap. Your guide will be carrying a mobile phone and maps to share with you during the tour. Trail running shoes are a bonus, but most routes can be safely negotiated with a good pair of road running trainers.
Beginners Guide to Trail Running - Great piece from TrailRunner Magazine in the US.
http://www.trailrunnermag.com/article.php?id=186&cat=5 and one from Runner's World US edition...http://www.runnersworld.com/cda/microsite/0,8032,s6-238-511-0-0,00.html. For a general running guide visit http://dealnews.com/pages/articles/beginners-guide-running/ (Jack - thank you for your recommendation).
Useful sites to visit...
Sa Milana (Alaro running club, recently crowned champions of the Balearics) - a great place to get a feel for what is happening in Mallorcan running http://samilana.blogspot.com/
Elitechip. This is the official site for entering all the mountain races, marathons and triathlons on the island. http://www.elitechip.net/index.php?zwshow=&zwlng=en
Council of Mallorca promotion of Drystone Route (GR221) http://www.conselldemallorca.net/?id_section=3198&id_parent=491&id_class=2992 (Spanish language, the English version is not available at the moment)
The walking bible of Mallorca from Cicerone http://www.amazon.co.uk/Walking-Mallorca-Classic-Mountain-Cicerone/dp/1852844884/ref=sr_1_1?ie=UTF8&qid=1321288837&sr=8-1 Also a guide to the Tramuntana Drystone Route GR221 http://www.amazon.co.uk/Trekking-Through-Mallorca-Drystone-Cicerone/dp/1852844957/ref=sr_1_1?s=books&ie=UTF8&qid=1321289083&sr=1-1