Over 100 volunteers have logged over 3,000 miles in the last four days searching for 79-year-old Elaine McKinley, who went missing from her hiking group in Portola Valley on Thursday.

McKinley, a Redwood City resident, was with friends and her 81-year-old partner of 34 years, Kit Durgin, when she rushed ahead of the group and disappeared in Windy Hill Preserve on Thursday afternoon. Unable to find her, and knowing she was likely confused due to dementia, Durgin and the group called the San Mateo County Sheriff's Office, and a search has been underway since Thursday evening.

As of Monday morning, McKinley still had not been spotted, despite the diligent work of 150 trained search-and-rescue volunteers.

Over the weekend, motorists on Highway 35 were told to be aware that search personnel would be active along the roadway.

"Over the last 72 hours, approximately 3,000 total miles have been tracked by searches on ground, either by car or on foot," the sheriff's office said in a statement.

"The San Mateo County Sheriff's Office continues to ask all residents who live near the preserve to check their cameras day and night, starting on Thursday 10 a.m. and to report this information though an online form," the sheriff's office added. "Any sightings, or lack of sightings, on camera footage will help aid in the search."

Embattled San Mateo County Sheriff Christina Corpus, who remains in office despite an effort by the county board of supervisors to take steps to remove her, was on the scene of the search over the weekend and spoke to NBC Bay Area.

"We remain hopeful," Corpus said, noting the specialized training of the volunteers doing the search. "They remain confident that they will find her."

Corpus added in a statement, "I want to personally thank our dedicated volunteers and personnel. They have not wavered in their commitment, and we are indebted to them."

Durgin said she was very worried about her partner last Thursday, especially because she did not have a cellphone.

"We were out there kind of to celebrate [that] we can still hike and we were enjoying the wildflowers and everything, and I don't know, at about three and a half miles, Elaine just took off like an Energizer bunny, which is pretty common," Durgin said, per ABC 7.

She added, tearfully, to KTVU, "I just think about what it would be like for her to be out there in the middle of the night, wherever out there is, you know, and probably confused."

The occasion for the hike was Durgin's 81st birthday.

McKinley was last seen wearing a red jacket, as seen above, dark pants, and carrying a backpack. Anyone who thinks they see her is asked to call 911.

Previously: 79-Year-Old Peninsula Woman Missing After Hike With Partner and Friends In Portola Valley