Basically everything from pick-up can be applied to long term relationships. Pick-up teaches you to boost attraction and keep lively conversations going. It teaches you how to build Kino. All of that transfer straight across. In fact, I would suggest that you start any long term attraction like any normal pick-up. Once the relationship starts, there are somethings you can do that you wouldn't during a normal pickup. Cliche dates (movies, dinner, ect) are no longer bad ideas. You no longer have to constantly be worried about attraction, and leading the interaction. Just hanging out becomes fun for both of you. Your end game changes to. The next interaction isn't any longer about getting closer to sex. Sex happens when you or she wants it, and there doesn't have to be large carefully constructed kino to get there. You just start in on the kino, and everything naturally will progress. Once your in a relationship, you should and can talk about feelings and things that are core to your personality. These type of topics have no place in pick-up but are essential to long term relationships. Another difference is that you don't have to be alpha all the time. Be careful though. Women are attracted to alpha men, and you should naturally be alpha most of the time. However, it is okay in relationships to let your guard down somewhat.
That's a starter. I would say in general, pick-up is valuable all through the stages of your life and relationships, but there are somethings that change when you want to enter a long-term relationship.