Although it was definitely not the longest walk ever, NEVER walk straight from the whiterun stables to solitude at a low level- you will eventually cross through the upper ruins of th Labrynthian, and run into at least ten frost trolls...
Anywya, my longest walk was from solitude to windhelm, south all the way to riften, west to falkreath, from there to riverwood, around the base of the Throat to Ivarstead, all the way up high hrothgar, the short way down (let's just say I am glad I had that half-damage from falling perk with heavy armor, and there where plenty of ledges, along with my knowldge of going down mountains- as in, have a lot of health, heavy armor, and angle intelligence.) From there, which was near riverwood, I went to Whiterun, walked around seven times for seven days and dragon-shouted, to see if the Wall of Jericho thing worked (it doesn't, by the way), and finally back to riverwood, and up the path to bleak falls barrow, then went the opposite direction from bleak falls to see if there where any caves. Don't ask why I did all that, cause it is a story longer than this one. -.-'