I would get there about 3 hours early. You still might be able to find a spot 2 hours out, but I wouldn't chance it. There are several chairs up on the train platform to sit in and tag as your very own . . . but only if you get there early enough.
One year I sat on the ground just behind the railing in front of the chairs and got some great shots! I had just bought a MK beach towel so that was my sit-upon! The people I sat in front of did not mind since they knew I would be seated the whole time. Worked out fine.
Even though 2-3 hours is a lot of "down time" it really is not that bad. I just relaxed, soaked in all the MK sounds and met some nice folks who were also waiting for the parade and fireworks.
This is also a wonderful place to take pictures as night falls. To see the stores on Main Street light up as night falls is just to incredible. You can also see below the CM's who are keeping the crowds entertained right before the parade.
See you there!