Alfie Bailey, 10, is taking on the challenge alongside his dad Geoff Bailey in aid of Severn Hospice, which runs hospices in Telford and Shrewsbury.
The Harlescott Junior School pupil, from Shrewsbury, wants to raise money in memory of his grandmother, Gina Bailey, who died around 20 years ago.
The pair, who will follow a route around Shrewsbury in April, have so far raised more than £1,000.
Geoff said: "I lost my mum 20 years ago.
"I've always wanted to do something for her. She died of a brain haemorrhage and Alfie never got to meet her so throughout this lockdown he asked me if he could raise money, in memory of her, for the Seven Hospice in Shrewsbury.
"I know a few people who have gone to the hospice. It's a good local cause."
He says the pair have been training in Shrewsbury and have run 60 miles in the past four weeks.
"We have a route that we are going to follow on the day," Geoff said.
"We will be starting in Sundorne and going over towards Meole Brace, and we will be finishing around the Quarry."
He said he is 'overwhelmed' by the generosity people have shown so far through their donations.
Geoff added: "It's overwhelming how people have gotten behind us. I'm so proud of my son doing it.
"It's a privilege to run with him.
"Alfie's a star who always likes to help others.
"He's football mad but he can't do that at the moment. We thought with it being lockdown, it would be an ideal time for us both."
Severn Hospice gives specialist care and support free of charge to families across Shropshire and north Powys who are living with an incurable illness.
To donate to the pair's fundraising challenge, visit the Facebook page called Geoff's fundraiser for Severn Hospice at www.facebook.com/donate/151202283486545/113711397392712