High precision NASA plans to prepare a marathon are here to give confidence...As cool as this story is, it really doesn't seem too bad to enjoy at least some of the digital assistance of the GPS/Smart watches - if you don't train for anything specific or to run a program towards a marathon with sessions in specific paces and distances - fair enough. If you run to keep healthy and go for example 10K a day - maybe the need to track it isn't as big
I personally don't believe plans that require to run 4km at 4.05 - 140bpm ...
It's false science