First mfm scheduled

My gf and I have been discussing an mfm since early in the relationship. She has done it before the relationship while I have not. It will be with one of our mutual friends from the gym, he actually reached out to us after seeing us on an app. Chill dude, very respectful. We both want to do it, and are really excited for it. One issue I have is post nut clarity, how it will affect me in the moment. Should we plan out how things go? Or go with the flow with boundaries already expressed? Would appreciate some advice