A trainer sounds like a great idea if you’re nervous about doing it yourself and when your horse gets the go-ahead from the trainer, scheduling some outings with a trusted buddy should be the next step… either with the trainer and one of their horses or with a horse buddy who has a calm and reliable trail horse and a good working knowledge of the trail system you will be exploring.
Especially as a beginner, know where you are going before you go there. Either get good advice from a friend, get some hiking/trail maps if the land is multi-user, or scout on foot if you must. It is miserable, and I do mean absolutely miserable, to discover that “Roaring Run Trail” starts out OK but turns into creek bed (rounded slippery rocks and LOTS of them) and your mare goes to her knees (at a very slow walk) twice in that distance but you’re kind of stuck because it’s in the state forest and there are no connectors – the only way out is through – and it didn’t look nearly that bad on the map. You will feel SO BAD. That was in 2007 and yes, the mare and I survived, but it was horrific.
It’s important, especially if you’ll be going out on your own, that your horse get some experience being “the first horse” and not just the “follow along” horse. The “leapfrog” game is very useful for getting a horse used to being in front, little by little and also schooling good trail manners.
In addition, if it’s possible in your situation, try to get as much “outside time” as you can manage for your horse, hopefully in a larger pasture with some terrain features (hills, a creek, some mud, just a little variety) and the potential for exposure to wildlife such as might be found in your area. You don’t want (for example) the very first whitetail your horse ever sees to be exploding out of the underbrush to your left. I mean, it can still happen that way, but… it would help if she’s seen some deer before.
And a safety consideration – carry ID on you, have a small dog-tag style ID affixed to your horse’s tack. It’s not perfect, but it’s better than nothing. Also, assuming you’ll have cell coverage, make sure your horse is OK with cell phones going off while you’re on her.
Trail work is not like arena work, but it is doable. It’s a different kind of hard.