You can watch runners in MK without a ticket.
For the other parks (also later in the morning) you would need a ticket to watch inside a park.
There are many great places to watch without having a park ticket.
I especially like to cheer for the Full in front of the Animal Kingdom entrance.
In front of MGM and on the Boardwalk are other great areas for the Full.
And the runners really need the encouragement near the end.
I never go to the finish area anymore. Way too crowded.
And now you have to pay $$$ to be in the bleachers closest to the finish line.
I'd rather cheer in several places where 'my people' need a boost! LOL