I’m an overweight, 68, male and I’m a bit faster, around 2.5 mph taking into account terrain. I actually work in kph because the maths are easier as OS maps are metric. I reckon on 5 kph on the flat in good going. Rougher country flat perhaps 4.5 kph, climbs 3.5 to 4 kph but it really doesn’t matter. I rarely stop for a break, eating and drinking on the go and resting by slowing down for a bit.
People were harder in Naismith’s day but you can tune his rule to your own performance. These days I can usually estimate how long a walk will take by eye. However, Memory map has a tuneable version of Naismith that you can use for route planning. The OS app has a crude version that usually overestimates route times.
As to ViewRanger, my advice would be to not bother pausing it. VR has 3 graphs, speed/time, height/time, distance/time. You can overlay all 3 to see how gradient, going and time affect your progress.
I have a look at the graphs in the pub just after the walk. After that I forget it other than to add it to my experience bank for planning the next trip.