I think when to move up depends a lot on the horse and rider in question.
To me, I read your description and think move up, no sweat. Your mare was going down the trail nicely, was taking good care of herself, and finished the ride in excellent shape.
However, if you don't feel at least 75% comfortable with that (are we ever 100% sure? I know I am not!), then you need to figure out what is making you hesitate and work on that. It was only her first ride, you went for a resaonable middle-of-allowed-time pace (which I think was a great plan!), so maybe next ride aim to finish just under the low end of the time and see how she does. If she again passes with flying colors, then move up.
I have known some people who have never done any sort of LD ride and went right to 50s. Hell, I have one friend who did a 100 as their horse's first ride (experienced rider though, just a new horse). There is no "right" thing for everyone. Do what is right for you!
Congrats on a great first ride. The pics show a happy mare and a happy rider. Love it!
There is no joy equal to that found on the back of a horse.