It has been precedent for a bit now that proof of time from a previous race needs to be no older than 2 years prior to race date (with slight wiggle room). This information is updated along the first information for the purpose of participants being able to enter a property dated proof of time. [If someone's was too old, then they would need to run a new race for a new proof of time and do it before the submission cut-off, not something everyone can do with a couple weeks notice.)
If runDisney offers early registraion, officially, you gain access via the AP holder page. Unofficially, you will find links all over (here, FB, the Google, etc.) I cannot say for certain how early AP registration is, as I haven't bothered in the past couple years--demand isn't as cut throat as it once was. I wanna say I remember a week before general registration.