The airport is relatively small, but also very congested due to ongoing construction. You can walk if you want to: from Terminal C you can walk to either the DL shuttle at Terminal B, or the US Shuttle at Terminal A - but if you have any luggage I would not recommend it. The walk to Terminal A would require you to go through the Central Parking Garage, and out the other side. Eventually they promise moving walkways but that is all still under construction. I'd say that walking briskly it would take 20 minutes from gate to gate. The walk to Terminal B is shorter but the current construction makes it tricky. Also the DL Shuttle is at the far end of Terminal B, so it also is a 15-20 minute walk. The free shuttle bus mentioned is the easiest way to go. Just look for the Blue and White Massport Bus marked Route 11. It stops at all Terminals (meaning you need to sit through a bunch of stops at Terminals D and E before looping around to Terminal A. Once you board the bus though you should be at Terminal A in 10 minutes. If you opt for Terminal B (DL Shuttle) note that the DL shuttle is the second of three stops for Terminal B.