If the issue is that you're afraid of becoming too tired to get back, I think ninerfan's got the right idea. Adding just one mile the next time you ride might be what you need to break that barrier. Then, when you're ready, add another mile, or even two. Plus, it's not like you have to ride straight through. You can always stop at your usual 10 miles to take a break and have an energy snack before pushing on for the extra miles.

And as for your mental barricade-- Think of distance the same way you thought about those hills you expected would kill you. How did you handle the challenge of the hills? Put yourself into that same frame of mind when you begin to add extra miles and I bet they'll end up being easier than you're allowing yourself to believe.

Hills are where I hit my own mental barrier, so I admire you for having conquered those!