Used to do a lot of that sort of backpacking in my late teens.
A full up load would be about 35pounds even with 1980’s kit (presumably heavier than modern stuff?)
First day used to be OK, the second day would be agony. After that it gets easier day by day. By the end of the first week you wouldn’t really notice the weight. I don’t think the sort of fitness required to run a marathon or ride a bike is that relevant. Its just getting used to putting one foot in front of the other for hours and hours every day.
The trick is to go moderate the first day, take it really easy on the second and then build your distance up again on third and subsequent days.