Moronihah does have trained and able troops, they simply were not stationed in the heartland. Moronihah hears of the fall of Zarahemla and the subsequent march of Coriantumr and he has a quick-march of his own ordered for the forces commanded by Lehi. Lehi’s force is already located in the north, probably in the fortified city of Mulek (Alma 53:2), which would be one of the next cities east of Bountiful. Thus Lehi was already in position to protect Bountiful, and Morohihah has him ready for the encounter when Coriantumr arrives.
At this point we have a fairly fresh and well-trained force of Lehi meeting Coriantumr’s force that has been on a rapid march, and doing some fighting along the way. Coriantumr’s men would have been tired from their march and efforts, and perhaps over-confident from the easy time they had to that point. When they met the organized and effective resistance of Lehi’s men, Coriantumr begins a retreat back up the valley. He likely intends to reach Zarahemla and use that defensive position to regroup.