The surprising thing I believe now after coming back as CEO of YC:

The size of your round doesn’t matter. Having 70 mo runway gives you a lot of shots but it doesn’t get you product market fit.

The thing that matters: you must be a great builder solving a real problem.

i'd echo this and also add: after the initial build period, if you're not getting traction in what you're doing you should pivot every year. 

12 months is more than enough time these days to realize if you are going to make it big, and if you're not, you should go all-in on something else

having 5 years of runway does you no good if you are doing the same thing over and over again. but it's 5 big swings if you manage it well