It appears there is a sidewalk from buena vista dr to the boardwalk. However, to get there from the speedway it appears that you need to do either one of two things if I follow this correctly:
- Walk out the front of the speedway out to buena vista dr and walk along the road or on the grass to get to the intersection with epcot resorts blvd as it appears the sidewalk has been removed for this section. you can then cross epcot resort blvd. At that point you would go straight on the sidewalk up to the fork where you would stay to the right and down the path to the walking path to Hollywood Studios. Or after crossing epcot resort blvd you could hang a right onto a sidewalk that will take you most of the way to Boardwalk Resort although it looks like you might have to go onto the parking lot for a brief spell around the security gate.
- Walk out the back side of the Speedway again on the grass or on the driveway and cross Epcot Resort Blvd by the Boardwalk resort sign. Hang a left back towards Buena Vista Dr on the sidewalk and then right at Buena Vist Dr and the follow the path to the fork as above. Alternatively, if you want to go to Boardwalk, once you've crossed epcot resort blvd just hang right and follow the sidewalk.
It looks like the picture from the OP was taken using this second option. It appears to be on epcot resort blvd heading towards buena vista dr with the speedway now on the left across the street.
Either way it appears that the trick is getting from the speedway to the epcot resort blvd. I don't see a sidewalk there from the front or back. But once you've crossed the epcot resort blvd there should be a sidewalk to both the Boardwalk and DHS.
Is that correct?