My wife and I have traveled up the north end three ways.
1. A gas-powered golf cart can take you pretty far (we went up about six miles past the cut, I believe). We rented ours from the place just south of the airstrip past the Palms, before you get to Island Supermarket. I don't think we were restricted to just Journey's End. I think we finally turned around only because the path was going on the beach and we weren't real comfortable with the sand traction at that point, if I recall correctly.
2. A bike ride is lots of fun if you're reasonably fit. We went up, in April, about eight miles past the cut (Tranquility Bay, or something like that, I believe). It's fun because it's slow paced, tranquil, and good exercise. It can get harder the farther up you go, because the paths are newer, and some times you have to ride on the beach, with loose sand. This was our favorite.
3. Hire a boat driver. Many of them didn't want to take us just for a ride--they wanted to do snorkel trips, which we didn't want. However, we got Chito Guerrero (sp?), whose number I no longer have but have heard his name mentioned in the posts before. I went to Karribean Water Sports and, when one of their own drivers couldn't take us, they called him. We went all the way up to Robles, and got to walk on the rocky shore where "the reef meets the shore." This option's a bit more expensive, but allows you to see the most. Plus, he'd stop anywhere we wanted him to.