I think there was an article in the most recent Bicycling Mag about which breeds are best for cycling (seriously).

Here's my experience.

I've got a Cavalier. She's got long hair. She loves to run single-track with me when I'm mtn biking. I dont' take her very often because with long hair (and the weeds on CA single-track), she's a disaster when we're finished and I have to cut weeds out of her coat. But, she loves it and would run herself into the ground if I let her.

Good caution -- dogs don't have the same control mechanisms we do. When we're tired, we stop. Dogs don't do this, so you have to make sure not to exhaust your dog. Build up her endurance the same way you'd build yours.

Now, totally unrelated to mtn biking, she loves to ride on my touring bike with me. Someday I'll get a pic. She sits in a crate on the back and we ride to the park and to the bike shop where I work. Need to find a little helmet for her!