Can we learn resilience? At 17 Marcus, known to his followers on social media as The Hull Boy, has lifted himself from the depths of despair 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
Dementia - A syndrome associated with memory loss and other declining brain functions.
Diabetic - Suffering from diabetes, a condition that causes a person's blood sugar level to be too high.
Depression - Low mood that affects someone's daily life, and can last for weeks or months.
OCD - Obsessive-Compulsive Disorder, a mental health condition involving intrusive thoughts and/or repetitive behaviours.
Tyson Fury - A British former professional boxer.