Hi There! Congrats on your new horse! Is she your only horse?
When I got my boy he was very VERY skinny too. I could see every bone in his body just about, and his hips looked very protruded. I started researching how to care for and feed him to fatten him up at his stage in life, and I had a vet out to meet him, examine him, and make recommendations. Personally I wouldn't get too worried about it yet. See what your vet says first.
Pictures would be a great help!
My boy is officially retired now. I was riding him until just a few months ago. So we're making the transition from riding companion to pasture companion. We go for walks together now instead of rides. And it's all good.
He SO enjoys his time out on walks, and I SO enjoy his company! Horses can be awesome companions! So if you find yourself unable to ride your new horse, please don't give up on her. You can still enjoy the great outdoors and get your exercise. There's ground work you can do, tricks you can teach her, walks to go on... All kinds of wonderful stuff, and the bonds of friendship and trust are well worth it.