‘Is a rule of thumb that helps in the planning of a walking or hiking expedition by calculating how long it will take to walk the route, including the extra time taken when walking uphill. It was devised by William W. Naismith, a Scottish mountaineer, in 1892. A modern version of this rule can be formulated i.e. as follows: Allow 1 hour for every 3 mi (5 km) forward, plus 1 hour for every 2000 ft (600 m) of ascent.’ Clearly the ‘rule’ is about young, fit people walking on clear flat terrain. If you are older or ‘bush-bashing’ you will have to apply some corrections.
‘It does not account for delays, such as extended breaks for rest or sightseeing, or for navigational obstacles…Over the years several adjustments have been formulated in an attempt to make the rule more accurate. The simplest correction is to add 25 or 50% to the time predicted using Naismith’s rule.’ https://en.wikipedia.org/wiki/Naismith%27s_rule
I think the rule is a reasonable guide for ‘track walkers’. Those of us who prefer more remote places will no doubt have worked out other ways of estimating. Doubling the time in much of the Victorian bush is reasonable. In off-track walks in Fiordland, forget it. There it will take you longer than you can believe to traverse a couple of kilometres!
The most important consideration is life is not a race. I have often encountered folks hurrying to their destination (ultimately death) who take no time to observe the wonders along the way. One of the advantages of being old is that it imposes a restraint such that you do have time ‘to smell the roses’.