When I try to go up the road to Fort Dawngaurd, it tells me "You cannot go this way". How do I fix this?

Not to be rude, but are you sure you're going to the right place? If you're heading to some gate at the south-east corner of the map, that isn't the way to Fort Dawnguard. You need to go through a cave in the Rift, sort of near Stendarr's Beacon.

Lulz. I'm a dummy. Thanks for answering my stupid question, though. That issue was not legendary at all.

Please look here too: http://www.redgamingtech.com/skyrim-dawnguard-location-of-dayspring-canyon-start-of-dawnguard-quest/