Lets take the questions one at a time.
Were the escapees and cops running all night? Why wasn't day break shown?
Its not uncommon for fugitives to keep running till they feel they have put some distance between themselves and their pursuers. And I bet the cops were willing to pursue them all night. After all there were some high value targets in the bunch like Burrows (the convicted killer of the VP's brother) and John Abruzzi (a mob boss). But do you think an episode dedicated to the first night of the chase would had added any value?
How did the scene change from a field to a forest all of the sudden?
Terrains change! Its not inconceivable to start from a point in the plains and ending up in the thick after a few hours of hike.