Just thought I'd add my experience...
I'd been in labour since 6am in the morning with horrible back contractions every 2 minutes lasting for a minute, so hardly a break in between. Also, I had spent the entire labour standing. By 11am we thought that we'd better get to the hospital which was an hour away in the car. I got in the car and got straight back out again - I didn't think there was any possible way that I could do it. Mum convinced me to get back in again and quickly drove off before I could get out! For that entire hour I gripped my Doula's hand and breathed and didn't open my eyes, somehow I had gone to some place within myself and settled into a rhythm where suddenly the contractions were bareable. When I got to the hospital I was fully dilated and just had the pushing to go.
I still don't know how I got through that car ride, especially at the stage of labour that I was in.
I think that when it comes to the crunch and you HAVE to do something you just pull up the strength from somewhere deep inside.