Two miles in 20 minutes is a 10-minute mile, which is a decent running pace. I can't imagine walking that fast. (When I'm walking fast, I'm around a 13-minute mile.) So if you were walking, it sounds like your route tracking might be off a bit. If you were running, though, perfectly reasonable. I'd expect about a 200-calorie burn in two miles, so 170 is on the low side, but not totally out of the question.