It is true that you need to get used to it, and there are certain things that will hurt when you are not used to a roadbike. Your back and your bum are two common ones.

However, pain becoming unbearable in just 14 miles is quite severe. And I would not advice you to just grit your teeth and bear it. The last thing you want to do is get an injury by ignoring signals your body is giving to you. Personally, I would start of with a less aggressive riding position. Make sure the reach is ok in terms of length from seat to bars. Also make sure the drop from seat to bars is not too severe (keep it level when you start off). I know the common suggestion is to "flip it and slam it" (referring to the stem), but most of the time if you do not have the natural flexibility, you should really do the opposite and ease into a more aggressive ride-style.

I have the same thing, and I know why. When I sit deeper, what I tend to do is actually straighten out my back. Instead of sitting on the bike and arching my back, it is more like I am "leaning into" the bike. The main difference is that I rotate my pelvis.