Swear Vengeance — F.M. Parker

Marie Hofer, a retired Army pilot and now a doctor, leaves her home and goes into the deep forested mountains of Pennsylvania searching for new herbs to test for medicinal values. On Black Log Mountain, she forces her aged body to climb to a high, isolated rock ledge. A giant oak tree has been struck by a powerful lightening bolt and blasted into splinters. At the base of the tree, Marie discovers a mutant plant specie. She samples a tiny bit of the herb's root to test for its effect. She falls quickly into a deep sleep. Upon awakening, she feels unusually vigorous. Over the next three days as Marie tests the root, she discovers that she is growing younger, wrinkles fading, beauty returning. She is amazed, and also frightened at what is happening to her.
Marie hastens to tell her long time friend Nell of the discovery of the miracle herb and to ask help in testing it. She finds Nell dead, murdered. When the law fails to find the killer, Marie swears vengeance on him and sets out on her own to discover who he is.
Marie, now young and with a young woman's desires, chances upon Brent Dickson in her search for Nell's killers. She falls in love with Brent. She does not tell him her true age. Brent has his own secret. He is being pursued by paid killers.
The conclusion of the story is startling, amazing.

From Swear Vengeance

Crocker gave Marie a defiant look. “I've said all that I'm going to.”
”You helped kill a friend of mine,” Marie glared down at the woman she straddled and held penned to the ground. The woman was a tough one and wouldn't break easily. What would be her weakness? Ah, that would be it. Marie took the Army Colt from her purse and laid the cold steel barrel on top of Crocker's nose.
“Now I've no time to play games with you. If you don't answer my questions I'm going to do something horrible to your face. No man will ever look at you again without puking. First thing that'll go will be your straight nose. If that doesn't get your attention, then your teeth get knocked out one at a time. Now I'm ready to start working on you. Anytime you feel like talking, just give me a signal.”
Marie raised the pistol above Crocker's face and prepared to strike. “To make you understand just how serious I am, I'll start by breaking your nose.”
“You're crazy!”
”Seeing your loved ones killed will make you do crazy things. Give me names before your nose is a bloody pulp.”
From Swear Vengeance