Pre-diabetes, I had no trouble running 6-7 miles at a time (3 times a week), but I can't comment on further/longer durations.
With that said, the initial 1-2 weeks were very difficult. Before my body went into ketosis, I had little to no motivation to do anything more than sit on the couch and watch tv.
EDIT- Those 1-2 weeks are called the "carb flu" on this website. Not everyone experiences it, but it's something I most certainly had difficulty with.