Don't worry! It can be hard to blend pesto evenly into pasta. One method that helps is thinning out the pesto with starchy pasta water. This recipe asks you to set aside 1 cup of pasta water before you drain the noodles. This water contains starch from the pasta, and starch helps sauce stick to noodles. I don't always need the entire 1 cup of pasta water, but drizzling in some as I toss the pesto and pasta helps the pesto blend in.