Finishing Strong

I am training for a 50K. Around 21 miles, I lose a ton of momentum, mostly due to leg pain/fatigue. I can make it the next 10 miles (I completed 29 yesterday) but begin to need walking breaks and "limp in". Can anyone help me take it to the next level? Given the amount of pain/fatigue, I cannot imagine how I could ever do a distance longer than 50K, and I think I would like to some day.

Thanks in advance . . .

A lot of it comes down to time on your feet - this should be priority over the distance. It always feels like a daunting task at first but you'll get there - good luck!

If it comes down to time on your feet, doesn't it necessarily follow that, if you want to go farther, you have to go faster? I am being told to slow down to avoid the pain/fatigue described in my original post.