As mentioned above, I think the NG Topo software is Very useful for planning days and distances.
I plot a route, add Waypoints for significant locations, and then create a route profile.
The profile gives me good idea of the elevation gains/losses, and from that I can better plan my distances. Other resources are invaluable for big picture planning, but when you cut to the chase, Topo is worth the effort.
One last place it shines is when you're out there and you want to reconsider your situation. With the waypoints, the elevations, and the distance provided on the profile you can easily reassess the ease or difficulty of making a change. "Can I push over the next pass?" "I'm ahead of schedule now, but is what lies ahead enough to set me back?" "I'm unexpectedly snow bound for 2 days. Can I finish the route with a push, is do I bail out here?" I tend to do long miles/days without much cushion. The profile give me the confidence to evaluate the options.