Its actually fairly quick, head down to the either the trains for the S gates or the N gates once you clear security. take the train to central terminal where you can cross over to the train that will take you to the lounge you prefer, and reverse when it is time to leave. The longest part of the walk will be getting from security (if you use TSA Pre or Clear) to the N gate train. Once you are ready to go back to the B gates just take the train to the main terminal (baggage claim) cross over and take the train that will take you to where you can transfer over to the S/B/A gates train. It lets out at the end of B, close to your gate.