There were several factors that really effected me when I suffered on my first hike of the Fundy footpath 10 years ago, after not doing much for 10 years. My friend and I hiked in to Dustin Brook from the access road, and camped. That was tough enough, but nothing compared to the wake-up call that awaited me.

1. Too much pack weight - I was at least 20 pounds over a good summer pack weight.
2. Too much body weight - I was at least 20 pounds over a good hiking weight.
3. Out of shape - Yeah, that too. Hadn't done much running in about 10 years.
4. Fever - I caught a chill paddling a few days before and was running a fever first day.
5. Hot - It was hot and muggy. I was drinking about a litre an hour first day.
6. Cummulative Elevation Gain - Fundy Footpath is all up and down.

First set of switchbacks leaving Little Salmon River is 300 feet straight up, and took me at least an hour, and that was just the start of the day. First day we covered about 11km in 11 hours finishing after dark. My hiking buddy was very patient. Second day 14km went better. Ditched some food, rather irresponsibly I might add but didn't know better at the time. Fever passed. Maybe a few less hills and less severe. Last half day was just a moderate trail and went much better, but was only 10km.

I've done it several times since in considerably better shape and with considerably less weight carried. High pack weight combined with extra body weight, poor fitness, lots of steep hills, and hot muggy weather is a bad combination. If you can still manage 10 mile days than its not too bad. If you can only manage 6 mile days in 12 hours of hiking, if you can still call that hiking, then you should probably have done things a little differently.