Danny O'Toole from Rugby has cystic fibrosis. He is 23, but because of his disease he is likely to die young.
This Sunday he will join thousands of other runners taking part in the London Marathon. He wants to raise awareness of cystic fibrosis, and raise money to help find a cure.
Despite having a disease which will eventually end his life, he maintains a genuinely positive outlook.
Danny says he is focused on keeping active and achieving his goals, and running a marathon is top of his list.
It's something I've always wanted to do to prove to myself that I'm normal. I've always done what I want to do – nothing's stopped me and this is another one that's not going to.
I'll do this and then I'll move onto the next challenge on my list.
Nearly ten thousand people have cystic fibrosis in the UK. Only half live to celebrate their 40th birthday.
The disease is inherited and is caused by a faulty gene carried by one in 25 people.
It means that as a result the mucus in lungs is thicker and stickier, which harbours bacteria which then in turn damages the airway.
Danny has already raised nearly £2,500 in sponsorship.