I do understand your worry. THis may have been suggested already but if you both have mobile phones, on the way to school if she walks part of the way she can text you to let you know she's safely arrived. LIkewise when she comes home, she can text when she leaves. Or whatever suits you. Mobile phones are like extended umbilical cords.
My 12 year old son never even walked home from his primary school just around the corner before, but now he -because he wants to- goes to school on the bus and comes home on the bus. I soon realised I was the only mum dropping off and bringing home a Yr 7 child, and he felt it too. He has been protected and cosseted all his life but now he wants/needs some independence. I have worked out what we can do to give him some independence, and he is meant to text me when he gets to school (which he often forgets) and text me when he is on the bus (ditto). There are busy roads he has to cross. I have told him to walk with his friends from the bus, and to cross the road in groups (on the assumption that a gaggle of kids is easier to see than a single child).
You can only become confident in her ability to do it if you let her. Trust yourself -you have presumably taught her to cross the road. And road safety, and stuff like being street-wise/stranger danger, etc. Ask yourself -what are you afraid will happen? Road accident? Abduction? These can happen to adults as well as children. Prepare her for the world, teach her how to keep herself safe. We ALL worry about these things.
And as for the school, remember back to when you made the decision. Why did you decide that one as opposed to the one across the road? If nearness is the only factor you will get over this, because she will cope with the distance if you can allow her to. And you must have thought about this when you made the decision in the first place -what factors decided you on one school as opposed to another. You probably made that decision when you were in a better frame of mind than you are currently in. You probably made the right choice at the time. And if you didn't, you are not married to that school! In the worst case, you can change, you can ask about transferring to the school across the road -and I know you'll say it's full and oversubscribed, etc, but people do drop out/transfer/emigrate etc. There may be a place. But in your shoes, I'd then worry I'd taken her out of a better school just because I had an issue with some things that she was probably taking in her stride!!! You can't win.
Give it time. Give her time to get used to it. Give yourself a break.
Finally, my DS is scared of dogs, phobic. To cut a long story short he had CBT, and his lovely therapist suggested his fears were about the world in general, but that because it's not possible to function on a level where you're scared of EVERYTHING, so we condense the fear and focus it onto one or two things -dogs, in our case. So his fear of dogs was masking a wider anxiety. You have focused on your daughter walking to school, and the choice of school. These feelings are normal. But if you feel yours may be a little more than normal, a bit more intense than they should be, ask -are they masking something different?
Good luck, and be kind to yourself today.