I'm in a similar situation to you, as I am a teenage girl and I have always loved hiking, but I've had some uncomfortable encounters with strangers while alone on nature trails. So I got a GSD for that very reason.

My GSD, "Bodacious," is trained in bite work, and he is a confident dog who would protect me if need be.

But yes, it does take professional training (and therefore money) to train in protection/bitework. Definitely do not attempt to train a dog in protection/bite work on your own without experience - mistakes can lead to turning your dog aggressive and dangerous to innocent people, which is not what a well-trained protection dog should be.

Another, simpler option is to simply train your GSD well in obedience. People will be wary of messing with you if you have a GSD by your side that properly heels, sits, downs, can do long downs, sit-stays and down-stays and is generally calm and under your control. In a lot of cases, people will just assume that a well-trained GSD in obedience is also trained to protect.

(You can also teach them to bark on command, which would likely scare off a person of suspicion)

I personally recommend seeking professional assistance in obedience too, and not just the training classes at your local pet store, but someone who is well experienced in training GSDs, perhaps someone who trains for schutzhund. I started out trying to train my first dog (GSD mix) by watching YouTube, but that only got me so far and left me frustrated. The very first day I started with my trainer, I saw immediate improvement in my dog's obedience.

My last recommendation coming from my own experience is to know what genetics you are getting in your dog beforehand. If you get a mix breed or "backyard bred" GSD with no pedigree or any way to trace the bloodline, you really do not know what temperament you are getting. I thought my GSD mix, Storm, would be a protective dog, because GSD makes up the majority of his breed mix, but that turned out to not be the case at all. Turns out he's weak-nerved and has no drive for training.

Bodacious, on the other hand, has Czech and German working line with a great pedigree, and his balanced, confident, and eager-to-please temperament with lots of drive is polar opposite from Storm lol. Not to say that a dog like Storm is bad - in fact he's the perfect picture of a "nice family pet" but I prefer a working line dog as my personal companion/bodyguard.

Working line dogs are high energy and high drive though, so they won't be the best fit for everyone. Research some different GSD bloodlines and talk with different breeders to see what will best fit you and your lifestyle.