Stone, 21, was eliminated on this week's episode, but when she returned home to Dittmer, Mo., her boyfriend Michael proposed.
"It was completely by surprise. I woke up with a dozen roses on my pillow and little love notes everywhere," Stone told reporters on Wednesday about the daylong proposal. "At the end of the day we got done working out and were getting ready to take our showers and go to bed and he said, 'Okay, Megan, I have one more surprise for you.' "
He told Stone to close her eyes, and when she opened them, her longtime boyfriend was on one knee, the bride-to-be said.
"Michael is a big romantic," Stone says of her fiancé, who started his own weight-loss journey when Megan left for the Ranch and has now lost 80 lbs.
The couple is looking forward to a September church wedding with fall colors, and Stone has already asked her Biggest Loser teammate Conda Britt to be a bridesmaid.
"She is the sweetest person ever, and I just love her to death," said Stone, who has been shopping for size-12 wedding dresses – a change from the size-20 dresses she was wearing just months ago.
And the big day will not be an excuse to cheat!
"I hope everybody that attends my wedding is ready to eat healthy, because there is not going to be anything unhealthy there except for maybe the cake," she said. "But I'm going to try and find a healthy cake alternative."