Lady Drusilla's Road to Ruin by Christine Merrill
MAD DASH TO GRETNA!
Considered a spinster, Lady Drusilla Rudney has only one role in life: to chaperon her sister. So when her flighty sibling elopes Dru knows she has to stop her! She employs the help of a fellow travelling companion, who looks harmless enough...
Ex-army captain John Hendricks is intrigued by this damsel in distress. Once embroiled with her in a mad dash across England, he discovers that Dru is no simpering woman. Her unconventional ways make him want to forget his gentlemanly conduct...and create a scandal all of their own!