Moronihah does have trained and able troops. They were simply stationed away from the heartland where he thought they would be needed. Coriantumr’s march on Bountiful brings his army within striking distances of part of Moronihah’s army. Moronihah orders Lehi to intercept Coriantumr. Lehi’s force is already 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 Moronihah has him ready for the encounter when Coriantumr arrives.
At this point, Lehi’s fairly fresh and well-trained force meets Coriantumr’s troops who have been on a rapid march and doing some fighting along the way. They would have been tired and perhaps overconfident. The clash of the encounter not only stops Coriantumr, but drives him in retreat back up the valley. He likely intended to reach Zarahemla and regroup from that defensive position (a distance of perhaps two hundred miles).