Quick question for folks that I didn't find asked/discussed on other Vietnam/Hanoi/Ha Long Bay threads.

My girlfriend and I are going to do a 3 day/2 night cruise at the end of May. We currently have it arranged with the shared vans to/from Hanoi. We're both avid trail runners and I was thinking that, if there was a good hiking/trail option somewhere between Ha Long and Hanoi it would be worthwhile to upgrade to a private car on the way back so we have the freedom to stop and check out a new sight. I looked at google maps and see a few rivers which might be fun to run along if there are paths to follow.

Has anyone done anything similar or know the area well enough to offer a suggestion? If the answer is that the whole route and everything around it is heavily populated then we'll just stick to original plan.

Thanks!