Thing is, Phil, 100 miles is a big deal... and then you'll find somehow it becomes something that you can think of doing again and again without problems.
It's a brilliant achievement and I remember feeling tired and achy after mine, although I was OK the next day (if a bit stiff).
Did another one yesterday and it was a different feeling altogether. I could have carried on for miles and miles more, felt normal when I walked into the house, and am about to set out on a twenty mile cycle ride to get some milk. No ill effects at all.
I think your body just gets used to it. And 100 miles feels like such a lot when you think about it - but once you've done it, you realise it is possible and even enjoyable. Good luck with your second century... and your third...