Everyone here is right, your dog is digging out of boredom. Long walks is a good thing, however that won't keep her from digging. She's a working dog and needs a job to do. Write her a job description and train her to do those things. The ONLY way to stop her digging without heeding this advice is to get her a kennel with a bottom so she cannot dig. But honestly that's not what you should do, you need to train her and work her brain.
You can start with some obedience, sit, down, and the all important come! Then go to some tricks, sitting pretty, BANG! (shoot her and she's dead) roll over, you can also teach her some useful things like to help make the beds, pick up garbage, ect. And prevention is the best medicine also. Don't allow her outside long enough to be come bored.