I'm the slow poke when I ride with my friends, but I always tell them they are not obligated to stay with me, I know they have their own goals, etc etc and don't ever want to hold anyone back. But I am on a road bike, and go to group rides and aware of mph, etc. You neighbor just has a drastically different idea of riding than you, which is fine...

As others have said, just be honest and tell her you are going to head out a bit earlier because you want to get some extra miles in, or you want to go work on your hills, etc. Maybe you could offer to ride with her when you get back from your solo ride? Or see if she can ride her bike to the coffee shop and meet you after your ride, and then you can ride home together.

Or you could just let the air out of her tires. KIDDING!!!!