Hey Mario,
You are correct, Marketo gives each Lead it's own munchkin code that is tracked on landing pages, emails, and links clicked.
It is my understanding that when you send the email, each Lead who does not have a munchkin code will be given one at this time. All you need to do is make sure the Form Prefill option is selected and you should be good to go. It sounds like it is by your example, but to ensure that it is, go to:
Admin -> Landing Pages -> Edit Settings -> Form Prefill -> Make sure it is checked
As with everything in Marketo, I would test first. Create a dummy record for yourself (maybe a personal email address, but completely delete yourself from the system first if you are in there), send yourself the email, and see if the information is pre-populated.
Let me know how it goes!