Ian and Jan Cummings are undertaking the arduous 80-mile walk in honour of wife/mother Beth, who passed away in April following a battle with cancer.
The pair left the northern tip of Unst at 10am on Sunday and they expect to finish up at Sumburgh at the end of the week.
By Monday afternoon Ian and Jan had raised over £4,600 for national charity Brain Tumour Research, with more donations expected over the coming days.
The intrepid pair had made it to the ferry terminal at Ulsta at 4pm on Monday after battling a deluge of rain in Yell.
They have been helped along the way by Ian's friend Tam, who has been driving with their camping equipment and food supplies.
Ian said the walk from Mid Yell to Ulsta was a bit "miserable", with rain nearly non-stop from midday onwards.
"We've actually made pretty good time," he said after arriving in Toft. "It took six hours to get through Yell.
"It's been good so far, but we've got a long way to go yet."
Ian added that after sitting down on the ferry to Toft, it was a "bit of a job getting up again - but once you get moving again you're alright".
He said a "good day" as they walk the Shetland mainland would see them cover between 15 and 20 miles.
Ian added that the amount of money raised had already surpassed expectations as Shetland continues to dig deep into the pockets for the good cause.
Community fundraising manager for Brain Tumour Research in Scotland Joe Woollcott said the charity was "extremely grateful" to Ian and Jan for their support.
"Sadly, Beth's story is not unique," he added.
"Brain tumours kill more children and adults under the age of 40 than any other cancer, and more women under 35 than breast cancer, yet just 1 per cent of the national spend on cancer research has been allocated to this devastating disease."
Brain Tumour Research is the only national charity in the UK dedicated to raising funds for continuous and sustainable scientific research into brain tumours.
To donate to the fundraising appeal, visit Ian and Jan's JustGiving page here.