are you riding alone? in my experience, it's all mental.
i would suggest taking how long it would take you to bike that distance if it were flat (call it n days) and adding a day and giving that as your estimate (n+1 days). but i would also try to make it in the if-it-were-flat time (n days).
for me, the pain of cycling long distances isn't about how far my legs can go in a day, but how far my butt can be in the saddle for a day. and while they're surely related, i imagine that it wouldn't be that different if the route were flat or hilly. when you go up a hill, you go slower, but when you go down you go faster, so it ends up being not too terribly different from flat, time-wise. sure, your legs get tired sooner, and you put more weight on your posterior, but i would try to make it in the same time. if it were me.
but on it being all in your head: if you don't embrace the hills and keep reminding yourself how much you love them, it gets a lot harder to say "i can go [so] much farther today." or if you don't have someone there to do that for you.