The advice recommends Naismith's Rule as a guide to how long a walk will take. However this assumes a walking speed of 3 miles per hour. Naismith was a very fit walker and I doubt an average walker would achieve this speed in the country, over stiles etc. Similarly, I doubt anyone who does not achieve the 3mph would achieve his ascent speed.
I have created a walking time calculator on my website, where I have tried to take account of this and would be interested in anyone's experience as to how well it works. Just Google "Walking Time Calculator" which should find it.
Let me know via email if you like - see contact page on website.