If it has not already been suggested, Chuck Wendig’s WANDERERS - it’s about 800 pages. And as a bonus, the follow-up is coming out shortly.

Wanderers by @ChuckWendig and it's perfect timing because book #2, Wayward, will be published in November!!! (Seriously, read Wanderers! I read it in a weekend (it's 775 pages) because I could NOT PUT IT DOWN!!!)