Hello! I answered a similar question a few months ago which [I believe is relevant] (https://www.reddit.com/r/AskHistorians/comments/3feqhe/was_there_ever_a_dropout_rate_for_roman/). Essentially, it is unlikely that recruits (during the Principate and Late Antiquity) would have been unable to handle the physicality of military life, as most recruits came from rural, agricultural backgrounds and were thus used to lives of intense physical labor.