I had a seven hour labour with DD1, and I think I walked at least that long in the hospital hallway! And with DD3, DH and I walked all over town trying to get labour to pick up. (But she was born at home.)
But the thing is, do you want to be committed to walking that long? And will you feel weird having to stop and pant and breathe and possibly moan on the way? Will there be total strangers coming over to you and offering you a ride to the hospital, since you obviously need to get there? And will you be OK walking the remaining half-hour if your water breaks after 15 minutes?
I think I'd say walk if you want to, but make sure there's a back-up plan available quickly! And I don't think you'll be putting your baby at risk by refusing to get in the car. I think if you know you need to be at the hospital quickly, or if you know you can't walk anymore, you'll be in that car like a shot!
All the best!