Best advice I've ever heard of came from (I think) was Willie Weir. Never base any of your major decisions (to quit or which way to go) at the end of the day. Get a good night's rest before making any decisions. I once cut a ride short because of a really hard day. I regretted it the next day when I was well rested. Second best advice is to pace yourself at the beginning. It's easy to push too hard at the start.