Easy but long progression achievement. Just keep driving! Your player profile tracks real-time progress in more detail than the Xbox achievements tracker. In normal play use
then
to see your global total. Note that this is in metres, so this counter needs to reach roughly 1,000,000.
You could theoretically earn this faster by shuttle-running highways, like the route from Smithville Dam up to the Drummond Island gateway, but that would be a tedious way to earn an achievement that should come as part of normal progression anyway.
I unlocked this after completing nearly all of Michigan and about 60% of Alaska, but your kilometreage may vary depending on how efficient your routes are.
The guide over on TrueTrophies suggests that this only counts when you're driving through mud, but I don't think that's correct. Mine unlocked in the middle of the asphalt on the Dam road in Smithville and the total on my player profile was roughly in line with the 1,000km. If it was mud only, I'd expect the number on my profile to be much higher, since the counter is definitely for total distance.
Let me know if there are any corrections necessary!