(This is if you are doing the missions) Once you get out of the tunnel, stick along the right side of the wall. Eventually you will come to a point where a fence meets a concrete wall at a corner. There, you can make stairs out of 5mm boxes to hop the fence and get into a courtyard. If you walk towards the runway from there, you will notice on your left are the outside walls of a row of hangars. Between two hangars is a stack of barrels. Place 5mm boxes as needed to jump onto the barrels, and then onto the hangar. Then, walk along the outside wall of the hangar until you reach the end, which is where a flat wall will meet the outside wall you're walking on, which is connected to the domed roof part. Build stairs with 5mm boxes from there to climb up to the top of the hangar. From there, you can walk along the wall on the other side of the hangar. From there, you can walk along the fence between the two hangars, and jump to the next hangar over, which should be the one right before the runway. Repeat previous steps to get over that hangar, and once you get on the other side, you can jump the barrier, and get onto the runway much earlier in the game.
Another exploit is in the building where the slow-burn flamer is, if you go out onto the roof, and jump to the highest point on the roof where the fan is up against the wall, you can build stairs out of 5mm boxes, jump up onto the roof, and walk along rooftops and the outside wall going around the base. Be careful of falls floors though, and save frequently. I suggest that, on any different part or pattern of roof you are about to walk on, that you shoot the floor with a gattling lazer, as anything that isnt solid, the beam will go right through, and anything that is solid, the beam will hit. From this point on the rooftop, you can actually jump the outside wall and get outside the map, where you will be able to walk around and interact with background props. If you continue to walk in any one direction, you will eventually meet solid flat ground with a constant texture. This will infinitively expand as you continue forward. If you wish to get back to the map at any given time, you can just simply fast travel back to any of the marked locations.