The key is to not overwhelm the child with technical information and to leave the door open for future questions. We should always answer our children's questions, using the correct terminology. They will absorb what they are ready to hear. Be ready to answer further questions as they come up. You don't have to focus on only the "big" talks -- you can always bring the subject up yourself as the situation presents itself, such as when someone you know is pregnant or the topic comes up on TV.
You should feel good that you handled this situation well and that you gave your son the information you wish you had received as a child. The fact that your son ran off to do "kid things" after your talk should tell you that he got the information he needed and that he felt comfortable with you in getting it.