I 'ran' it yesterday.
Raven Rock to Tenpenny Tower.
10.5 minutes.
Naturally I didn't have the facility to sprint in FO3.
More generally, you don't hear people complaining that the Witcher III map is too small.
This evening I ran and swam (including some sprinting) as Geralt from the Niflgardian Camp to Novigrad's temple District in (topically) 11 minutes.
After doing so more testing and finding Geralt shares Fallout 3's 5.7 metres/second jogging speed,
I've been able to compare the estimated Fallout 4 map and Witcher III's Velen/Novigrad maps to scale:
https://dl.dropboxusercontent.com/u/87106516/fo4_tw3.png
Actually, displaying the Witcher III map upside down shows how much the two maps have in common,
including the proportion of water.
People thought the Witcher III Velen/Novigrad map was huge (and much of it is swamp and woodlands)
the density of Fallout 4 with a larger city mass than Novigrad will be quite a thing.
I was certainly concerned initially at reports of a small map,
which I'm now satisfied is utter twaddle...