Your feelings are absolutely normal. My wife and I started running about 2 years ago August (she made me!) and we signed up for the 2024 Springtime Surprise Challenge and 5k before we were really in shape. We did a few local races (5k, 10k) that fall, probably trained too hard, I know I always tried to go to fast. I developed some pains that I still deal with. We did ok with the SS races, completed them, and learned quite a lot. We used a run/walk routine to complete the 10k and 10 mile. Since then we have learned a ton about training correctly - the biggest thing was to slow down and do most (80%) runs at a slow pace, this really helped with endurance and being able to go the longer distances. Since that SS (2024), we have done all of the WDW races, all distances, completing all but the Marathon, because my wife tripped on a traffic cone, fell and broke her wrist at about mile 20. I'd like to think that we would have finished the marathon, but it is now our main goal - to complete the Dopey Challenge this time (and yes, we have a lot of nervousness about that!).
The weekend challenges do seem daunting, but one thing I noticed about the Disney races is the unbelievable atmosphere of being there with all the other runners, sharing that experience, and all of the things on the courses - characters, music, gospel choir!, etc. It gives an extra lift of confidence. You never feel alone. The best feeling is going around the final corner, seeing the finish, then getting there - feels like winning the Olympics!
Another thing that has helped a lot is the community of people here on the boards, sharing their experiences, advice and encouragement.
I am sure that you can do it.