Can we learn resilience? At 17 Marcus, known to his followers on social media as The Hull Boy, has lifted himself from a difficult place to meet an extraordinary challenge.
Teenager youngest to run length of UK
Change: Marcus, pictured above, always made excuses to skip sports day at school — but now says running has transformed him. Glossary
Hull - A port city in East Yorkshire, northern England.
Dementia - A syndrome associated with memory loss and other declining brain functions.
Depression - Low mood that affects someone's daily life, and can last for weeks or months.
Anxiety - A feeling of unease or worry. It can be mild or very severe.
OCD - Obsessive-Compulsive Disorder, a mental health condition involving intrusive thoughts and/or repetitive behaviours.
Tyson Fury - A British former professional boxer.
Mental health - The part of your health that has to do with your mind, your thoughts and feelings.
Resilience - The quality of toughness and flexibility that enables you to bounce back after setbacks.