I went last year and rode with Iguana Mama who are based in Cabarete. They are quite pricey but they do a range of rides.
http://www.iguanamama.com/
Most people start with the half day downhill ride which is quite undemanding and then there are several longer more severe ones.
Because there are so many Americans go there they do the whole "wrapped in cotton wool" thing. "No seramos Americanos" is a valuable phrase to learn. (We're not Americans). When we went, a minibus took us and the guide to the top of the mountain, and then after breakfast we started gently riding down the road, with a few stops in gravelly lay-bys (with the bus following). We realised that the guide was checking us out for skill levels because as soon as he was confident we weren't all talk we then diverted down some nice rocky paths and ended up wading across rivers.
Tips – take your pedals and make sure they swap the brake levers around the right way.