There are almost weekly (organised) walks run by the Departament de Medi Ambient i Habitatge. I don't think they publish the agenda on the internet but they do have a booklet that details all the walks, dates and contact numbers.  the booklets are available in many places, "Guia d'activitats del Parc Natural dela Ports".

This sunday, 18th October, it's La Mola i les Foles, difficulty medium/easy, all day - take drink and food. Starts at 9am. Information: 977 500 845. You need to call to let them know that you are going and they call if there are any changes.

The walk we went on a couple of weeks ago that lasted about 6 hours had over 50 people attending, from children to senior citizens. It had some quite challenging sections walking along 'caminos de cabras'

The walks are free and most seem to start with everyone meeting up at the Els Ports office in Roquetes (as you leave Roquetes going towards Els Reguers/Alfara de Carles the office is at the junction on the left hand side).

Brian, he could try to ask eurorob but perhaps it would be better to ask ebrorob

Colin i have just sent you a PM

You can also try here but you have to pay the membership, just 20 EUR per year

uecdetortosa.blogspot.com