Proud to say I accomplished that! My dad trained a fair bit. I did like 1 10km walk the week before we started, and I finished the 800kms with no real dramas, except the first day which was a massive climb. After 5-6 days my body was totally accustomed to the task at hand.
(Super irrelevant to the post but it was an awesome experience and I would advocate anyone to undertake it, regardless of your thoughts on religion. I am not religious but did find it spiritual in a way. I did it about 6 years ago and still think about it every week.)
Others have offered fantastic advice, I’m not going to add anything from personal experience. I will share this link: https://fs.blog/david-foster-wallace-this-is-water/
Reading this isn’t going to change your life, but I find it very helpful to reframe how you think about life. It deals more with the long term impact of having a full time job, outside of work in a way that worked for my brain. Hopefully you find it helpful but it’s not a single solution.