Claire Allen, along with sisters Lindsay and Nicola Scott, have taken up the challenge in order to raise money for Marie Curie Cancer Care.
Claire said: "We all really admire the work that Marie Curie do, so we decided to support them. We're hoping to raise £1200. It's the first marathon any of us have done, the training has been really tough on us and our families. I've always wanted to do a marathon, I just didn't expect it to be so soon!
"The furthest we've run in training is 23 miles, so the first time that we attempt 26 miles will be on the day. I'm just looking forward to crossing the finish line now!"
The trio have been following a tough training regime for the past four months to prepare for Sunday's event, including taking part in the Stonehaven half marathon in the summer.
Both Claire and Lindsay are leaders in Stonehaven's Jog Scotland group, which meet twice a week in the town, and Claire attributes her involvement in the group to her forthcoming achievement.
She said: "Jog Scotland has brought me to where I am now. When I joined three years ago, I had never run before. I think that doing the Loch Ness marathon is a pretty big achievement, and it gives us confirmation of everything we've done and all the hard work we've put in."
The Baxters Loch Ness marathon, which takes runners around the shores of the famous loch, is part of the Festival of Running, which also includes several 10k and 5k fun runs in the area on October 5.