The United squad returned to training a fortnight ago and are currently in Australia as part of their summer tour.
"It has been tough. The first week back in was pretty hard and, after the first couple of days, a few of the lads were struggling to move," the centre-back told the club's official website.
"But that's what it's all about and we want to get as fit as possible before the season."
The Premier League champions will take on an A-League All Stars team on Saturday afternoon.