This is a nice bit of analysis from The Monkey Cage. It links to research that looks at the question of how predictive of election outcomes polls are by how far out from the election we are. You can see the whole report here.
The upshot is: current polls likely mean little, except to the degree that they inform the dynamics of the pre-race: who is able to raise money, who is able to command media attention, and the like.
As I have pointed out before, if you want to know who is likely to win a party nomination for president, early polls can help a lot, at least for Republican candidates: early polls tend to be more predictive for Republicans when it comes to winning the nomination, and less predictive when it’s Democrats seeking the nomination. But for the general election, 300 days out is still a long, long way out.