What I want to know is how in the hell did they get the herd 20 miles out in a day, without eating bathroom breaks, etc. At best those walkers move at 2 mph meaning a twenty mile walk would take 10 hours...

 

If they were out at the pit first thing in the morning (say 7am) the Walkers broke out. They had a total of 24(?) miles to walk (a few miles before getting to the "and 20 more" point).

2mph = 12 hours. 7am-7pm. If it's summertime, it'll get dark around 9pm, giving a couple hours to play around with the new group before it gets dark.

If the walkers can walk 2.5 or 3mph, then it takes even less time.

By time it's dark, Rick can do the math and know the trio should have been back a couple hours ago. This is why he and Jessie are getting concerned.