To cross back over to America from Mexico on foot, you will need to get into one of the two lines…read more The left line is for those with a regular passport, while the right line is for those with global entry. That right line is never an actual line. It's like when Disneyland first came out with the Fastpass; you get to just walk all the way to the front without stopping. If you do not have Global Entry, or Sentri, then you could wait anywhere from 20 minutes, to two hours on foot. If you're entering by car, it will always take at least 2 hours or more, unless you have Global Entry, or Sentri of course.
On foot, the line to cross the border is outside. The line is covered, but you will see many Mexican residents walking up and down selling things and asking for money. It is always best to keep your belongings as close to you as possible, and not have anything sticking out of your back pockets for easy access to be stolen. But as long as you aren't being flashy, and you're not showing off expensive jewelry or waiving your money around, you'll be just fine. I do wish that the line was inside the building, like how it is for the line on the other side when you're waiting to get into Mexico. Once inside though, you'll just be asked two questions, and then you're welcomed back in.