Walking and resistance...

SweetHoliday

How do you calculate in resistance as you walk? I was walking on a dirt road with lots of rocks that slowed me from a 15 min mile to an 18 minute mile. Also, how do you add in the extra couple mph that are not in the mfp system? I was going 3.2 mph not 3 mph. Help! I'm sorry if this is already a topic...I just have no clue how to search for it.