Well, as Parla has pointed out the obvious answer is, "I'd fly" ;)
But (much) more important than that is the grammar of your question. It's not usual to use "First Conditional" sentences (If you do x, what will happen?) for situations that are not likely to happen. In this case it would be better to ask:
What would you do if you could fly?
or:
What would you do if you had the ability to fly?
or:
Where would you go if you could fly?
Because no matter how optimistic you are, and no matter how many Anthony Robins- like books you read, it is still impossible to fly like a bird ( or at least not very probable!)
Compare:
1.
A: I hadn't studied and I answered only of the questions in the test. I'm definitely going to fail.
B: What will you do if you fail?
2.
A: I did great! I answered all the questions correctly!
B: Well, what would you do if you failed?