I agree with all the stated above. I will only add some more points.
The overall picture is ominous for he characters. It must be in this way and it will a very important part of your job during the session to create such a feeling in your players. The great numbers will be against them and the tactical map (if anything similar is available for your players) will show no safe spots. But you are running/playing a roleplaying game at a micro-level. Here is where they have their chance.
First, an encounter with enemy forces does not mean that the group is spotted. Probabilities can play in favor of your players. If they are moving on foot and are few in numbers they probably will spot the enemy before being spotted. Second, avoid the usual temptation to simplify the terrain where the characters are moving. Don’t limit yourself to the short description of an encounter table. They are in a battlefield with ruined farmhouses, wreckages of both sides, smoke, corpses, abandoned material, and all kind of stuff… add it to the normal terrain characteristics that could provide some concealment to the group, things like irrigation channels, little bridges, fields, woods, rocks, bushes, stonewalls… Take some maps or aerial pictures to help you to describe the terrain. This thread may be useful:
http://forum.juhlin.com/showthread.php?t=566
If they are wise, they move in short jumps, carefully studying their next position before moving.
Other resources will be given or suggested to you by your own players. Enemy uniforms or vehicles? Known minefields to protect their movements? Some kind of help (forced or voluntary) by civilians? Do their understand the enemy radio communications?
And finally, others are trying to do the same that they are doing and the situation will be extremely fluid. Actions by other groups of allied soldiers could act as diversionary facts to help the movements of the group.
Good luck for your game! Keep us informed!