MG/YA Realistic fiction. Boy with illness is running away (?) to climb a mountain (he's defying a rule). Some pages count down the distance to summit, top of mountain. Read around 2014-2018.

[Lenna]

I read this book around 2014-2018, but I'm not completely sure. Possible spoilers ahead:

There's this boy (er, I'm pretty sure it's either a preteen or young teen) who has an illness, and I know he's defying some rule to climb this mountain. There are certain pages within the book that says the distance to the peak of the mountain. Can't remember if it was measured in miles or kilometres. I don't think he manages to get to the top, but I can't be certain.

Thank you very much for your help!

[Rainbowheart]

Hunter in the Dark?