If you aren't sure that your bike is set up optimally for doubling your current distance, is there any reason you can try a 15 mile ride first? Do you need to jump up to 20 right away?

I would think that if your bike is potentially an issue, then you may want to ramp up more slowly, just to be sure. If it's too painful at 15, then you know that you'll have to invest a little $ before you try 20. Anything that hurts at 10 miles is only going to hurt much worse at 20.