Threadmill lies to me pretty much all the time. In most cases it doesn't even work for me. Unless you have a medical condition, it is very unlikely that you'll get 180bpm from 5km/h walk (mine usually don't go above 150 even going uphill with 20 pounds weight vest on my shoulders). 180bpm is 3 hearbeats every second - pretty easy to check with finger on your wrist.