Well, it doesn't matter. For all I care, they can stay in my mountains all they want, my cavalry (or panzers) will be waiting for them just outside.
I don't know about enemy roads not giving a bonus to speed. That's stupid at any rate. But even if they don't, that's only an advantage for you. You can move your troops faster than they can, thusly being able to plan your defense in advance. I don't see how you could need overwhelming numbers in order to provide effective defense.