You're talking about halfway the length of the campground, I'd guess about half a mile. 20 minutes maybe? I've always biked, golf carted or taken the internal bus so can't answer exactly.
the bike barn is in about the middle of campground. as pp have said it is about half way thru campground. near main pool and campsttore in center of campground. bike barn is between store and pool somewhat hidden if you are not looking for it. from what i remember not seeable from main loops but not hard to find when looking for it