Processional/recessional Song?

You can look into Country Music Awards nominee lists from the past few years. Take a listen to some of the songs and see which works.

Do you know some of your fiancee's favourite musicians? If you really want to surprise her, pick a song from one of those artists instead of Dan + Shay or Ed Sheeran since they're such popular wedding staples. Of course, if she likes Dan + Shay or Ed Sheeran, go for it but unless she actually really like songs like From the Ground Up, 10,000 Hours, Perfect, etc, it may not feel as personal or special because they're played at so many weddings.

You want to make your future wife surprised to a song she would not expect, 10,000 hours seems slower paced to walk down if you do like that song. A suggestion if that helps to match your fiance choice.

Ask your fiancee (assuming she's the one who's making that decision). It would be good to know if they're walking as an instrumental or a cover instead of the original recording.

I think Mairi's Wedding is fine for a recessional. Something upbeat as everyone leaves the altar would keep the energy levels up.

Do you know what your bridesmaids are walking down to? If you do, find a song that compliments the tone of that one to help make the procession feel less jarring. Of course, if your favourite songs are stylistically different, you don't have to follow what they're doing.