That's right, close to 2 minutes clipped off my best time. What was different? First, I ran yesterday, so today defied the conventional "train every other day" wisdom. But the every-other-day thing is intended to promote cross-training, so that a) stressed tissue gets a chance to heal and b) the little oddball muscles that don't get worked in your main exercise do get some challenge in your cross-training. But for me it's turned out more like, run one day, don't exercise at all the next. At most a couple sets of wall push-ups, crunches and squats - low-intensity, and short duration. The alternating-days thing does not mean "don't do cardio every day", it means "if you do cardio every day, then alternate the forms of cardio exercise" so you're not running your knees off or swimming your shoulders off or biking your... knees off I guess.
Second, I hadn't eaten at all; most weekdays that I get to the gym, I start the day with black coffee, and grab an egg+cheese and another large coffee with cream en route to the office, and by the time I get on the 'mill ~4-5 hours later, presumably the carbs are past my stomach and my lower gut is working on the proteins and milkfat. You'd think the caffeine would boost performance, and if I was younger maybe it would, or if I was caffeinating up only 30 minutes to an hour before exercising, but the way it actually clocks out I think the caffeine-adrenalin-insulin pathway has scrubbed my blood sugar by the time I'm actually hitting the 'mill. Today all I had was my black coffee, and a few swigs of Tang about an hour before I reached the gym.
Third, I took my half-lap warm-up slower than usual, an easy 3.3mph walk. I've experimented with faster warm-up speeds, up to a moderate 4.1mph power-walk, but it seems all this does is prematurely drain muscle energy stores. I doubt I would have taken 4 laps at a jog, much less 6, after a brisker warm-up. I'm a little concerned that I might not be warming up properly.
And fourth, the gym itself was on the cool side, and I had a treadmill in the path of an AC blower, so heat stress wasn't as much of a discomfort as it can be sometimes.