1. Thinking you are a legend!
It's great to be confident and give something a crack but I believe some people bite off a bit more than they truly understand, perhaps overrate themselves a little even, they think they can do it on a little less than will be required. If you have experience or background you will often be realistic and confident. Perhaps the tendency to tell everyone they can do anything they want very young is responsible. We are capable of a great deal but often the reality of the work and prep needed is a surprise to people. I say yes, go for it but if you don’t have a big background, give it some respect and know a lot of training and learning is involved. It's worth it for success! So, prepare really well, think about the detail. Practice every possible component... and reduce the risk of an early finish and airport arrival on crutches or in a chair.
2. Not getting the most from your Coach
Perhaps people do not realise but a Coach can give you a little or a lot. They will get to know you and your training ethic and willingness to learn or execute races and that will tell them at what rate they teach or approach new levels. So, develop that relationship, provide feedback (we need it), complete training and show your work ethic. Only together will you get a shot at your goals so get the most from your Coach. The more he/she feels you are keen and learning and demonstrating an understanding of the training set then the greater chance of success and higher levels of learning.