16 minutes per mile (3.75mph) (5 miles in 80 minutes) is basically my 'fast walk' time in the city. My wife who is shorter has a hard time keeping up with that.

Unfortunately stop lights/etc. will also make this longer. My "best" pace where I actually do get tired is about 15 minutes per mile. Few can keep up a 4mph pace even on a dedicated walking trail with no elevation changes for any material length of time.

Most folks average probably greater than 20 minutes per mile, especially for longer walks. 2.5 hours is definitely on the extreme end, but close to 2 hours is probably more realistic for most.