I can't tell you what my real pace is because the route I take has so many stops. Although the computer on the bike stops when I do, it doesn't compensate for the slowing and accelerating again after stops. That being said, my 40 mile ride yesterday averaged 15.5 (actually 41 miles). It's a fairly flat course. Of course, this is riding alone. I'll have to time myself on a non-stop loop sometime and see what I average. Of course, when I ride in a group and draft, the pace is much faster.
On your 100, see if you can get with other riders at your pace. I usually have to eat something if going more than 40 miles (for my 40 I just drink water, but take an electrolite tab before and after--it's mostly sodium but different kinds). Lots of great books on this. The way to improve times is to do sprints like when running.
Sounds like many of the other folks on here are giving great advise. The only way I get in trouble is when I get competitive if someone passes and I decide to keep up <g>. If I ride alone, I pretty well let my body tell me what it can do and stay within that.
I can get shin splints if I don't really stretch my calf muscles well. Also, sometimes when sitting or lying down, I put the heel of one foot toward the front of the other and lift using the muscles in front of my shin to strengthen them. Seems to do it for me.
Best,
Dave