I fear I should point out that we don't really have any better way to measure the over-road distance marched than using a source like Google Maps. The source does add "little detours" (which usually means moving down roads that were not straight - the Urbana Pike is not straight and is a historical road - or filtering through the streets of a town) but if we use Google Maps consistently for all measurements of distance we can at least presume that most errors would be in the same direction. On the other hand, using the straight-line distance from start point to end point will inevitably produce a lower estimate, because even if the straight-line road exists it would only admit ca. 30,000 troops per day - this is how many troops can fit down a road in one day.It's hilarious to zoom in on those routes. Did they really take all those little detours? I doubt it.
If all of McClellan's army took the shortest straight-line route from a camp at Tenleytown to the middle of Frederick, then the length of the route would be about forty miles; however, it would take three full days to fit his army down this road. This means that at a march rate of ten miles per march his army would start marching on the 9th (say), his vanguard would march on the 9th-12th inclusive and his rearguard would end their march on the end of the 15th (being thirty miles behind) - for an apparent rate of march of only ~6 miles per day.
Moving in columns along parallel roads increases march distance but reduces "traffic jams" and this is why armies did it; a representation of march distance that does not include this is inaccurate. In fact, if desired I can examine a specific scenario of the ones I looked at above which does consider marching along parallel roads, and this would likely slow McClellan's army down somewhat.
This is why when looking at the distance Jackson could consider his troops to be able to march in a day based on recent experience, above (where I cite the movement from Harpers Ferry to Boteler's Ford) I used the modern road network and caveated it as such; if Jackson was actually able to take a more straight route with less in the way of the little detours forced by the modern road network, his estimated march speed before straggling to bits would be lower.