Its a 4~ mile journey on the route that he could not be seen. The average daylight walking speed on a flat surface is 3.1 mph. When you include Naismith's rule/Aitken corrections (rough surface and night time) you have a speed around 2.1mph so you do the maths.
Besides Julie said that Jeremy said he said to her that he used a bike. Since her words are gospel truth from an angel in your perspective why does this even bother you?
Yes maybe the bike was used.
I'm not sure that your reliance on rules and averages is sound.
I live exactly 1 mile from town. It should take me 21 minutes by your calculations, I do it in 13 minutes at a brisk walk, or when out running I do it in 5-6 minutes.
The various routes from WHF, are mostly made up of gravel farm tracks rather than rough ground. Applying somebody's mathematical equation sounds really clever, but I don't think it's particularly relevant.