We Do Not
I'll start off by saying your answer appears to be based on a false premise in that Gared/Will didn't actually outrun the White Walkers. In the show it appears to let Will go as it is right next to him when it kills Gared. However, in the books it appears as though the Others aren't even aware that Gared was around as he stayed with the horses and wasn't near the other two when they were killed.
Game of Thrones
The fastest we see a White Walker move is actually in the prologue. There's some camera trickery going on here but they appear to be running faster than Gared/Will are through the snow but aren't moving faster than a human should be able to. This sets their lower bound at around a humans running pace.
You'll note from the scene that when the White Walker kills Gared it appears to spare Will. Will sinks to his knees and seems to accept his fate, however, the walker throws Gareds head at Will and doesn't seem to approach him. The next we see of Will is at the headsman's block. So it seems likely he was spared and didn't actually outrun them.
A Song of Ice and Fire
We know even less of the Others in the books. However, I was able to find this quote which implies that they are good runners but doesn't really give any information as to how quick they are.
The wights had been slow clumsy things, but the Other was light as snow on the wind. It slid away from Paul's axe, armor rippling, and its crystal sword twisted and spun and slipped between the iron rings of Paul's mail, through leather and wool and bone and flesh. It came out his back with a hissssssssssss and Sam heard Paul say, "Oh," as he lost the axe. Impaled, his blood smoking around the sword, the big man tried to reach his killer with his hands and almost had before he fell. The weight of him tore the strange pale sword from the Other's grip.
A Storm of Swords, Samwell I