I just got back from a short trip to Lisbon, Portugal. I ride a road bike here in the US and I originally wanted to find a company that would support a day long road ride. However, it looked like most companies do mountain bike tours only so I went that route.
I went with a company called Lisbon Bike Tours run by Felipe Palma. My friend and I booked it via email and he was pretty cool. The weather forecast was mixed and we didn't want to go if it rained so he simply told us that he'd come over to our hotel if the weather was good and we can pay him on the day of. If it rained, he would just stay home and we would cancel the event.
We got lucky and it was the only day of our trip that did not rain.
He picked us up at our hotel, drove us up to Sintra, and fit us on our mountain bikes. The ride itself was AWESOME! As I stated earlier I'm not a mountain biker so I wasn't really used to all of the dirt and mud and sliding around, etc. Since it had been raining the ground was pretty soft and muddy. We also had to deal with puddles, fallen trees/branches, etc.
We were in the mountains so there was considerable climbing as well.
But overall, it was an awesome event and I would recommend it to anyone. However, this is definitely NOT a ride for beginners. Felipe is cool and will take it slow if you can't keep up but there is a lot of climbing and maneuvering. I would say that you should be at least an intermediate cyclist who is comfortable riding 20-30 miles of hard terrain and climbing. I generally ride about 60 miles on my road bike on a ride but it's mostly flat with occasional hills but this was more difficult. I was beat at the end of our 30 mile ride.