That 777 should be pretty close to the center of the B gates, so a 100 yards or so to the train, ride from B-A and LH leaves close to the train in the center. You can do it. I wouldn't want to be in the back of the 777 or have it run late.

I hate to miss LH legs because I am never sure how well they will accommodate me. By then I think you are too late to try to get to another hub for a TATL.

You are just one small hiccup away from missing it.