My brother (peterpoddy) can ride all day at a decent pace and never seem tired.
I'm not that fit. I'm also heavier than you right now, probably by a stone or more. But there's only one real reason I can keep going all day: I pushed myself above and beyond anything I though I could ever do on the Trans Wales. That's it.
Basically it taught me 2 things –
1) I learned what I need to eat and drink to keep me going without bonking or cramping (I start to cramp above 30 miles or so without proper fuel)
2) I trained for it, but nothing prepares you for 7 50-60 mile days in a row, in proper steep terrain, until you've done it for the first time. I got to a point on day 5 (My low point) when I doubted every single minute that I could carry on for one more minute, let alone more then a day. And now it's simply a fact that I know what I can stand, and I know what my pace is, so I can stick to it for a very, very long time.
🙂