The youngster's spot-kick was saved by Simon Moore after the play-off tie finished 4-4 on aggregate, sentencing the Robins to another season in League One.
But McCormack believes the 19-year-old, inconsolable at the end of the match, showed a lot of guts to step up to a highly-pressured situation.
"It takes a very big man to stand up and take pennos," he told the Swindon Advertiser. "I'm not comfortable doing it, there are a few of the other lads who weren't comfortable doing it.
"Miles was comfortable taking one, unfortunately for him the keeper guessed the right way. He took some during the week and he seemed to be pretty good at them.
"He'll learn, he's only young, he's 19 years of age, he's going to have more days like these and - if he plays to his ability - he's going to have a lot more days where he has celebrations instead of defeat."