You're going to have to be very patient in this mission--don't be surprised if you spend 12+ minutes in it. The safest approach is to consult your map constantly and save often if you're committed to one of the lower difficulty levels. When you see a guard, keep as much distance as possible from him.
At other times, in the more wooded areas toward the end of the mission, for example, get in sneak mode and hide behind the trees. You'll have to slowly work your way around the tree to keep it between you and the guard as he passes. When he's finally gone you can run for it until you've met another guard. Remarkably, I didn't have much trouble in these missions, and I never used the tunnel or the trucks for transport either. Needless to say, this is one of the more difficult sneaking missions of the game.