Q: If I take in 2000 calories a day and run 3-4 miles a day. How many pounds will I lose in a week

A: That's not enough info to answer the question. But assuming you usually eat the same 2000 calories to simply maintain your present weight, 3-4 miles/day every day of the week will lose you about 1 pound per week of bodyweight.

BUT one pound of bodyweight lost is not the same as one pound of FAT lost. Please read this before you continue with your program: http://www.mikesfitness.com/content/burning-calories-muscle-cardio , http://www.mikesfitness.com/content/cardio-does-not-destroy-muscle-when-... and http://www.mikesfitness.com/content/weight-training-better-than-cardio . There you will find why cardio (walking, running, etc.) is NOT the great fat-burner you may think it to be. After you read those, please check out my Nutrition article and Weighttraining article and perhaps reconsider your program. Good luck!