Personally, I would gone immediately. That said, it was a high stress, emotional, and scary situation. What's done is done, there's nothing that going back now would accomplish. If you really feel bad enough to go back and explain what happened, I would suggest a letter instead. Let's you get it off your chest, let's the owner know what happened, and doesn't risk a physical altercation because the owner is upset about you not stopping immediately.
If it makes you feel any better, I've had 3 dogs hit by cars. The first survived, the driver did not stop. A neighbor told us. The second was lost and it took me hours to find him because the driver did not stop. It was devastating, and frankly, I've never gotten over his death. He was small, though, and it was night. I work off the assumption the driver thought he was a opossum or raccoon or something.
The third was hit twice, both times the driver stopped. Both times the driver was just as upset as I was, and we comforted each other. It wasn't their fault. It was my fault for not staying in top of fence maintaince, or assuming because I live in the middle of nowhere that's it's safe to let them run. Most dog owners aren't going to blame you, unless you're driving recklessly or aiming for the dog.
You're okay. It's a horrible situation, but you didn't do it on purpose, you didn't do it out of malice. You're still a good person. Be nice to yourself and try to move past this.