Hi Stan,
you will get a lot of different opinions on this topic. The real answer is yes, you can walk to the start. The other question though is do you really want to?
Couple of issues I've heard of. The traffic, and others will chime in, but I hear the traffic can be pretty bad because some of the other roads are closed, increasing the traffic on this road. Also, you cannot use the bag drop because you come in from the back and do not pass it. Not an issue for a lot of people. Otherwise those are the only complaints I've heard. Again, others will chime in.
The next question is why would you want to? Now this is just my opinion. The way I look at it is that I have trained for months, got up real early to do the long runs, spent many hours at night alone on the roads to get here. I've spent way too many hours getting ready for this event, why would I want to shortchange the experience for an extra 90 minutes of sleep?
This is a destination race, not just another local marathon. I come for the whole experience. To miss the staging area with the bands, excitement, meet and greet with all my friends from the boards, well that is just missing a lot of the experience. Again, not everyone will care. We all usually congregate in the staging area before the race. Great time to calm the jitters and catch up with people you only meet online. This is what actually brought me back for 1 more Goofy.
As for sleeping in, that is so overrated

I'll get up for the 5K on Friday, then the half on Saturday then the full on Sunday. An extra 90 minutes of sleep is not that big of a deal. Heck, I can barely sleep anyway. I shower and shave before I go to bed so I get up at 3:30 then catch the bus around 4:00 ish. By the time I am at the staging area it is around 4:30 then we start making our way to the start around 5:15 or so. Time really flies and the gun goes off before you know it.
As you can tell, there is no right and wrong way to get to the start, just differences in experiences. I like the pre race excitement and wouldn't miss it for the world. Walking in the back way to the start for me would be like missing a large part of the race.
I know that is more of an answer than you wanted

Whatever you decide, good luck!
Duane