But, as the name of the tournament implies, the race has just begun.
Walshe shot a 6-under 65, her best score in her four years on the LPGA Tour, to take a one-stroke lead Thursday in the opening round of the tournament formerly known as the Jamie Farr Toledo Classic.
Jessica Shepley of Owen Sound, Ont., is among three players who are tied for second. Teenager Lexi Thompson and Paula Creamer, who won the tournament five years ago, also shot 66.
Top-ranked Inbee Park, winner of the year's first three major championships, is within two shots.
Walshe, among the top 20 on tour in putting stats, three-putted at the first hole for bogey — then took only 19 putts on the next 17 holes.
"I was like, 'Here we go again,'" Walshe said. "Then I one-putted the next hole and got my confidence going."
She also chipped in for birdie on the 14th hole, her fifth of the day, to jump-start her round.
Walshe came into the Marathon, in its first year under its new title sponsor, quietly. She hasn't had a top 10 this year and ranks 55th on the money list with earnings of just over $120,000.
She hasn't finished higher than a tie for 17th at Kingsmill. Her career best is a tie for eighth a year ago at the Kraft Nabisco.
"I've been hitting it probably the best I've hit it in a long time this year," said the 28-year-old University of Arizona grad from Westford, Mass. "I've been giving myself a lot of opportunities, and it's frustrating that I haven't been able to really go deep and make some putts."
Everybody in the 144-player field was tormented by temperatures in the 90s, high humidity, little wind and a glaring sun. Thompson said after her round that she drank a bottle of water per hole to stay hydrated.
Walshe benefited from a morning tee time, but it wasn't as if it was cool then, either.
"At 7 in the morning I was sweating and I'm sweating now," she said just after completing her round around 1 p.m.
Creamer, who opened with a career-low 60 in her 2008 victory at the course, was satisfied to be a shot off the pace.
"I'm very pleased. I had a lot of shots out there that I could have gone lower with," she said. "But at the same time, it was a good start. I haven't been able to start off the way I wanted the last couple of events, so this was nice to post a good number."
Shepley, a former University of Tennessee player, has struggled this year but is encouraged.
"I've had a couple of good results. I played a couple smaller events up in Canada," she said. "I've been working hard on my swing a lot. I'm trying to stay patient, so was hoping one of these rounds would come soon."
Thompson, who was the youngest winner ever of an LPGA event when she took the 2011 Navistar at the age of 16, was asked if American players had been eclipsed by the South Koreans who have dominated in recent years.
"There are a lot of good American golfers out here. There is so much competition," she said. "It doesn't matter where you're from; it matters how well you bring it out on the golf course."
No one questions how well Park has played. She'll go for an unprecedented fourth consecutive women's major in the same year when she heads for the Women's British Open late this month.
She's already won six times on the tour this year while ascending to No. 1. Naturally, galleries expect her to win every time out.
"I do feel more pressure coming into this tournament," she said after a 67 that left her tied with Haeji Kang, Jacqui Concolino, Gerina Piller and Karine Icher. "But I try to enjoy it. I enjoy the fans coming out to watch."
Defending champ So Yeon Ryu had a 68 along with a large group that included Morgan Pressel and Natalie Gulbis. Se Ri Pak, a five-time winner of the tournament, opened with a 69 — the same as the world's top-ranked amateur, Lydia Ko.
The course is built for low scores. Over the last six years, the winners are a combined 103 under par.
"I know there are some very good scores out there," said Ko, who appears a lock to run her streak of professional cuts made to a perfect 22 for 22. "I've just got to play my game. I can't control what the others do."
Rebecca Lee-Bentham of Toronto is in contention with a 2-under 69. Maude-Aimee Leblanc of Sherbrooke, Que., is another shot back. Alena Sharp of Hamilton turned in a 72. Charlottetown's Lori Kane and Sara-Maude Juneau of Fossambault, Que., had 73s. Sue Kim of Langley, B.C., had a 4-over 75.