Hiking speeds (as opposed to walking or strolling) depend on terrain, the weight you are carrying, weather, fitness, training, stops for water, rest, photos, etc.
What's the difference between hiking and walking? I guess people define it for themselves. For me, hiking point-to-point in Scotland means getting to your final destination of the day where you can find a dram or two of whiskey. In other words, it is longer and more intentional. The main thing is not to stop for lunch. A stupid granola bar or candy bar is fine, but for a 10+ miler a lunch will take all of the wind out of your sails - blood flow all goes to your GI tract for digestion. Our readers who have done our 10-12 -miles NYC urban hikes know what pizza and beer do to you at the halfway point.
Why not drive? You might well ask. I guess I do it to save the planet...of course. Don't you care?
Average hiking speeds are around 2-3 miles/hour, but what good is an average? We've hiked in Scotland with a 75 year-old lady who hikes about 4 mph, and she is over the hills and far away by the time you tighten your shoe laces and put on your rain gear.
For 6+ mile hikes, Mrs. BD and I tend to average 2.3 mph including brief stops. Hills or no hills, she tends to keep a constant pace while I lag a bit on the hills.