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.)