The one that got away (Last)

This week I finished my book, The one that got away by Chris Ryan. In this book the topic was being stuck somewhere with no food or drinks and needing to walk. The theme was never give up no matter what. Chris Ryan represents that in this story by going 5 days without any food or drinks with no friends around or any hope to get to the friendly border/ territory. This shows that you should never give up no matter what. There were parts where I didn't understand like how did a random stranger appear behind him? Also there were these other times like he spoke in a really advanced level of English so I did not understand some parts. I wondered why he chose this topic instead of all those other things he could write about. I would want to learn more about why he was sent there and why only 4-5 people came with them.
Thank you for reading,
Kaan