For my part, she is too young to ask for speed and/or endurance. She is still trying to find her legs
Were she mine I would work on keeping her gait consistent for the period of time SHE is comfortable with
As she gets older she will be able to hold her intermediate gait longer, but Gaited Horse #1 is not always capable of holding the gait (and hold it at speed) as long as Gaited Horse #2.
My step-pacer is only 14.3H. I retired him from neighborhood gaiting races, unbeaten, when he was 16. Every horse he beat was bigger than him and some were 16H and really athletic
Point-being they don't all have it in them for a lot of speed for extended periods.
That all being said, if she is having trouble holding her gait at a speed she is comfortable with and continually breaks into a trot, I would also consider having the chiropractor look at her.
My then 3-1/2 yr old suddenly couldn't hold his running walk for more than 30 feet and I noticed he was trotting way too much in the pasture.
It turned out that his Atlas bone (head/neck area) and his sacrum (butt area) both needed adjusted. I was instructed to give him a week off, but it ended up being two weeks.
When I did ride him again, I gave him his head coming home. His running walk was smoother and faster than before and he had a beautiful head nod. He felt so good that I had to make him dog walk because he was still too young to gait the two miles up hill to the house.
Hope this helps some