You should get lots of good answers. But I suspect the problem is the same as anyone. Wanting it all at once and too soon. If you are at 13 mph average after 100 miles, that seems very good. I am at 500 miles and have only averaged that on one ride. However in those 500 miles I have gone from a slow 8.5 mph to 12+ in 4 months. And my cadence has gone from at 65 rmp to about 72. It's just a matter of spending time on the saddle.

What makes it fun form me is changing up the routes. Not doing the same thing over and over again. And going to new paths on saturdays for my long ride.