That backfill info is much appreciated Steve. If you've not been above 12K' prior, you'll likely find that the additional 2500' of elevation slows you down more than you might expect. The 12-13K' level seems to be the "wall" where many hikers begin feeling the effects of AMS, conditioning being completely irrelevant. You'll be above 13K' for quite a few miles, and around 14K' for a surprisingly long period. I doubt if your pace between 12K' and 14.5K' will be the same as you are used to below 12K'. This is very, very common for hikers of all ability ranges, so don't be surprised if it happens. You might consider adjusting your start time to take this in to account, especially if the weather forecast is unsettled. Most Whitney dayhikers try to start about 2:00 am. A second night of sleeping high before the hike might help your acclimatization tremendously if you can squeeze it in. The nearby Horseshoe Meadow campground is at 10K', significantly higher than the Portal, and many choose to try and get at least one night there.
Weather and the trail: if it's rain, no biggie if you're properly geared - just expect to move slower and be a bit miserable. If it's snow up high, watch your footing along the backside of the crest. Nothing inherently dangerous in the trail design for an experienced hiker, but it does narrow in a somewhat gnarly fashion and has pronounced exposure. You'll be above timberline for about 12 miles total on the hike, and rock + snow can be a dicey combination. A fresh snowfall turns many people back along that last couple of miles between the JMT junction and the summit - it feels dangerously above their pay grade, especially if hypoxia/AMS has jumped their back.
Bottom line, be careful of your pace. Enjoy the hike and try not to make blazing up the trail your priority, especially since this is new territory and elevation for you. An aggressive approach to this mountain has flamed out many a peak-conditioned marathoner well before the summit. Best of luck. Looking forward to your TR.
