You CAN walk from all the terminals at Logan. It's not easy, but possible.

Starting at E, you have to weave through walkways that are a cross between sidewalk and roadway to D (the fake terminal, just State Police here), and then outside D to the AirTran check-in area. Interior corridors take you to C, and onto the AA part of B. You can cross the street to US B, or walk on a sidewalk/roadway combo to A.

Alternatively, from D, you can take the walkway to Central Parking/Hilton Hotel and onto A.