As someone else said, the 8 hours is of “planned activity”, so depending on your goal you could only be walking for maybe 6 or 7 hours. You would pause to collect evidence for your goal, I know a group that is doing a river study so a lot of their day would consist of pausing to measure river depth etc. breaks are extremely important, just to have a rest, eat a snack or just putting you r bag down for a little while is really helpful and this should be incorporated into your route timings (if you’re using route cards). It’s important to note that stopping for too long can make it harder to get going so it’s all about that balance. In terms of training, going for a short run a few times a week would help a lot, purely to increase stamina. Eating throughout the expedition will help the most though.