From the Force Awakens novelization

I love the fact that Phasma, who is 6'7 and built, is visibly intimidated by Kylo but Hux stands his ground.

So I'm not sure how widely this is known in canon but there are separate academies for those entering the stormtrooper program and those who intend to become officers. This is more prominent in the Empire. But the divide exists in the First Order as well. Stormtroopers can rise up through the ranks and take on a role as officer, much like Kath in Black Spire, by Delilah Dawson. But that is fairly rare. That being said, Hux was never a stormtrooper. Hence the tidbit about him not having any real military combat experience. He certainly has military combat experience from simulators.

But beyond that, Hux may have real world fighting experience from his adventures with Grand Admiral Sloane on her journey to and within the unknown regions. The First Order was still heavily under development. Sloane not only needed support from surviving Imperials, but also funding, resources, and weaponry. Thus, she would have to forge connections with allies still located in the known galaxy. Not to mention, Sloane had to survive as as Imperial fugitive until she, Brendol, Armitage and the teen soldiers were able to find the Eclipse. Being far less susceptible to suspicion, Armitage and the teen soldiers were probably the ones tasks with on world supply runs. I'm sure that Armitage encountered danger more than once while traversing these uncharted parts with the rest of the group.

*I say teen soldiers to illustrate the age gap. Considering the children were all preteens & teens while Armitage was 5. They are all still children, regardless.

Okay but, consider that The First Order was still in the early stages of development when Armitage Hux would be attending Junior Academy and later the First Order Academy. His junior academy experience would have been one of the first trials for the First Order junior academy and was most likely on a star destroyer, in the wilderness of some remote planet, a tundra, or an underwater facility to avoid detection. Probably one of the two former options since the First Order had limited resources at the time. Whereas, they might have established an undwater academy ( much like the underwater weapons facility featured in Bloodline ) by the time Armitage graduated.

And honestly, Hux probably didn't feel the need for military combat experience after learning guerilla style warfare while surviving in the unknown regions with Grand Admiral Sloane & contributing toward the creation of The First Order. I wouldn't be surprised if Armitage went on supply runs as a child ( before his intiation into junior academy ) because he would be the least suspected, considering the New Republic was hunting Imperial war criminals such as Rae Sloane, Brendol Hux, and the other former Imperial officers they recruited during their travels. Organized military combat must have seemed so mundane after fighting to survive.