The Wedding Journey by Carla Kelly

A shy surgeon, Captain Jesse Randall has long been secretly in love with Elinore Mason, the daughter of a drunken officer deep in debt to Major Bones, who is determined to seduce Elinore.
The British Army has begun its winter retreat from Spain, controlled by Napoleon, back to the safety of Portugal. Orderly retreat is the plan, with each of Wellington’s innovative Marching Hospitals protected by a regiment as they move out.
Not if Major Bones can help it. He will use this opportunity to ruin Elinore Mason, who assists in a marching hospital. Her mother is dead. Nell’s father, the hapless Captain Mason, is useless in a crisis. It’s Major Bones’ perfect opportunity for mischief of the worst sort.
But here is Surgeon Randall, determined to save Nell’s virtue. He can only do this by marrying her. This act of mercy (and love) brings down the wrath of Major Bones on every doctor and patient in Marching Hospital Eight.
Now begins a desperate race to the border. Is the shy surgeon any match for someone as evil as Major Bones? Will love triumph, or ruin?