Complex sentence: The campaign made a strong impression on Harris, particularly the long desert marches--some three decades later, he wrote that "to this day I never walk a step if I can get any sort of vehicle to carry me".
Split into simpler sentences: The campaign made a strong impression on Harris, particularly the long desert marches. Some three decades later, Harris wrote that "to this day I never walk a step if I can get any sort of vehicle to carry me".