I did completely overlook Soulcasting. I read the first 3, and will read the whole of that thread later.
But actually your edit is wrong, and especially in a medieval time frame, armies on the move stretch for miles if not days, except for the very brief moments when they bunch to attack one another. With soulcasting you alleviate some of that but the Alethi seem to be importing matériel and manufacturing it at the camps, and then with wounded and troop transport you'd still have long lines, nearly unbroken lines most of the time.
Navy's bunch up, like I think you imagine armies doing, but even modern armies have this problem, even with an airforce. Which is somewhat like what the Alethi have, with the spans(radio) and soulcasting(air lifted supplies). Think of the long times that the bridgemen rest for in the book, transporting only minor, mobile strike forces. Then exponentially expand the necessary transport time and space needed.
I've heard the largest tank battle since WW2 described as really, really bad traffic for hours punctuated by a few minutes of excitement.