Let me see if I can try to explain. Maybe
@katyringo can get some images of it tonight.
If you look at a map, you'll see that
Disneyland is set up like spokes on a wheel. The center is the Partners' Statue (Walt and Mickey) and the lands jet off from there.
Below are two images. Fantasmic is on Rivers of America. You will sit where the red X is. The red line shows how you'll exit through Adventureland. When you get out of Adventureland, you can go in whatever direction you want. The masses will be exiting the park, however, many stay to go on rides until park closing. If that is the case, stay to your left when you exit Adventureland. When you get to the hub (circle around the statue), you can go in whatever direction you want.
On Main Street, there will be cast members with red hand held lights which will direct you towards Main Street. The white hand held lights (at least last night) directed you towards the walkway behind the stores. This was not intuitive. They just kept shouting and telling people to move. Really, they should have been explaining what the red, white and green handheld lights meant.
The walkways behind the stores are on both sides of Main Street. They use them in situations like this and to move people up and down Main Street during parades. Even if you end up in one of the walkways behind the stores, you will not be end up outside of the park. You will be just outside City Hall. To get back in the park you'll either have to swim up stream or take the train to ToonTown Station.
Edit: Alternatively, as
@disneychrista mentioned, you can turn to your right and walk towards Galaxy's Edge. From Galaxy's Edge, there is a walkway to Frontierland which I would avoid because it will take you back to the Fantasmic crowds. There is another walkway into Fantasyland. See picture below.
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