Luckily, you don't need to actually WALK that distance, any distance travelled on foot counts, be it walking, sneaking, running or sprinting. If you don't just rush through the main questline, but also like to do sidequests and explore, you should get this achievement way before completing the main storyline, there is no need to refrain from riding or fasttraveling. I usually ran (= standard movement) or sprinted to train stamina within settlements and when leaving the paths in dense forest, where riding is awkward anyway, and used the horse when travelling on paths outside the settlements.