Just for fun:
I was able to get 838 mi in Google Maps by (for some reason) using I-270 and I-170 to bypass St. Louis to the north, then backtrack on I-70 toward downtown. I have no idea why anyone would do this, though.
I managed to get to 1,697 miles to Denver (to be precise, the Colorado State Capitol), but only by "bypassing" nearly every city between Baltimore and Denver using 3dis and using I-68 to I-79 rather than the PA Tpk, then using I-25.
Same approach gets me to 2,200 mi to Cove Fort (specifically, the end of the ramp to I-15 SB), but the route goes through Kansas City rather than using a 3di to bypass it.
These aren't great, but hey, I was bored (waiting for a computer process to finish running and I have nothing else to do!
). But it does show that while the estimates are close (they're just the real distances rounded up to the next 100, except for Denver), it would take some routing weirdness to get to those locations in exactly those distances.