I decided to avoid doing anything more lengthy than an NBME exam because I imagine the adrenaline rush one gets while actually taking Step 1 will help overcome any lapses in concentration or boredom one might experience during a 7 hour exam.
Even if you took a full-length simulation and found that your mind wandered all over the place and you performed more poorly on the last blocks than the earlier ones, what are you going to do about it now? Behavioral therapy? Ritalin? Seems like just another discouraging factor one doesn't need at this point in the game.