Matthew Eager, a student at St Oscar Romero Catholic School in Worthing, pedalled from Littlehampton to Goring-by-Sea railway station and home again.
He said his route avoided the main roads, so he had to cycle on all terrain, and halfway through, he passed his school, where teachers showed their support.
It was all his own idea, to raise money for two charities that are close to his heart.
Matthew was raising money for CLIC Sargent, as his cousin Lewis was diagnosed with leukaemia at the age of three, and the Royal Marines Charity, as his dad was a serving marine for 15 years.
Mum Rebecca Eager said: “We’re all incredibly proud of Matthew. This charity ride was entirely his idea as a personal challenge and a way to show some support and gratitude to the charities that have supported us in one way or another as a family.
“He first came up with the idea at October half term but because of lockdowns, etc., it’s taken him until April 3 to complete it - which happened to be two days after his 12th birthday.”
Matthew has so far raised £241 for CLIC Sargent, which fights for young cancer patients to ensure they thrive, not just survive. Click here to support this charity
And he has so far raised £185 for The Royal Marines Charity, which supports royal marines and their familes to overcome their challenges. Click here to support that charity
Matthew said: “Keep safe, and thank you all so, so, so much. It’s been incredible the amount of support.”