The highest I've ever been up to is 100.
I find that 70-80mpw is no harder than 30 (importantly, 70-80 is not leaving me sore and exhausted. 100+ would be a different story). In terms of motivation, though, it's all about the first few miles. It's not that much harder to run 10 miles than it is 5, but it's a hell of a lot harder to walk out the door knowing you have to run 10 than it is to walk out expecting 5. And getting out the door in the first place, even if its just for 2 miles, is the hardest part by far.
Motivating myself to get out the door, especially in the morning, is something I still struggle with. The three ways I address it are:
1) Think about how hard I've worked, how much I want to be better, and how much I shoot myself in the foot if I don't get out that door. The only way this works as motivation is that I've skipped enough days to know how shitty it feels, and not want to do it again.
2)Make getting out the door a habit, and never think about it. In high school, motivation was never in issue because it was just what you did after school--classes, lunch, classes, run. If you make it a part of your day and never allow yourself to think about it, it's easier. NEVER hold a debate about whether or not you should run in your head. It shouldn't be a question.
3) Have a buddy. I can't tell you how many times I've woken up, cursed my running partner, plotted his death, and dragged my sorry ass out of bed so I wouldn't be late. There's no way in hell I would have gotten out of bed a lot of those morning without a commitment to him.
Good luck!
"Success consists of going from failure to failure without loss of enthusiasm"--Winston Churchill