The Brown family completed the iconic distance as they took on the 2.6 Challenge to raise funds for the organisation which provides vital support to them.
Sam Brown has Morquio Syndrome, an ultra rare, life-limiting condition which causes shortness in stature, progressive physical disability, fatigue, pain and complex health problems.
The condition makes walking and running difficult for the youngster.
But it proved to be no barrier for the brave 11-year-old who took on the marathon challenge with mum Katy, dad Simon and eight-year-old brother Alex outside their home in Otley.
Katy said: "It was great fun and we got a lot of support from people.
"Sam really enjoyed it and completed the last lap. I think it was good for him to have a little challenge to push himself."
The Browns' efforts have so far raised £1,800 for the The Society for Mucopolysaccharide Diseases (MPS Society)
There are only a hundred or so people affected by Morquio Syndrome in the UK.
It is one of a group of metabolic conditions called MPS.
The MPS Society is the only national charity in the UK supporting families like the Browns, providing advocacy, information, raising awareness and funding vital research into new therapies and treatments for these incurable condition
Katy explained: "Sam has a restricted airway and a history of pneumonia, so he is at very high risk of Covid-19, and as a result is being shielded. We cannot leave our house or garden.
"So daily walks and runs are not possible and we’ve been running laps around our garden instead."
Simon and Katy both completed the 2018 London Marath to raise funds for the MPs Society.
This year's race should have taken place yesterday but has been postponed for the first time in its 40-year history as a result of the coronavius pandemic.
It is hoped the race can now go ahead in October.
The 2.6 Challenge aims to get the public to come up with activities to compensate for the cancellation of major mass sporting events because of the pandemic.