While the task was hard even for experienced Elf trainers, it was gruesome for a HUMAN to attempt. Especially when the novices were probably old enough to be Aragorn's great-great grandfather!! The Man already knew they would not listen to him even if their very lives depended on it. No matter how he tried, he would probably end up being the token third commander of the joint army. And in Aragorn's mind, there was definitely something more worthwhile for him to do, like hunting orcs or killing wargs or destroying spiders or maiming Nazguls or annihilating wolves... Yup, Aragorn's favorite pass-time was venting his pent-up angst of being in exile on innocent, unsuspecting creatures of darkness.