My rule of thumb for planning purposes in most conditions is 3 km per hour, or about 1.9 mph. That gives plenty of leeway for the unexpected (heavy terrain, heavy pack, unfit person in the group or the group just feels like strolling about, chatting and having a good time).

A fit person can comfortably walk as fast as 5-6 km per hour (3.1-3.7 mph) on very easy terrain (firm paths, relatively flat). I don't think it is realistic to plan for anything faster than that. Your mileage will vary, but 6 km (3.7 mph) is where I stop enjoy walking. (Depending on company and situations I sometimes just have to stop whining and endure a faster pace than I like). I have friends with longer legs that comfortably can walk faster, but they are a minority.

So - most of the time I use 2 mph, but I'll use 3.7 as an upper limit if conditions justify it (paved path, easy terrain, known participiants).

Running is a totally different story.