If you feel confident about the marathon register now. My first year with Portland Fit I waited until I completed the 21 mile benchmark to register for the marathon.
There is no Portland Fit discount for the Portland Marathon.
The good thing about registering early is that you will be more committed to the training program and you won't wait too long to register and end up paying a late registration fee. Once I put it off and had to pay $125 to register late.
Another good reason to register early is that you will have a lower race bib number

Michael