don't see much need for warmup or cooldown over 2 miles. I think you have to treat the TT like a race day. That 15 mile week is the final week of the season.
I am a large fan of the Portugese protocol of long warmups and cool downs before races and workouts–it's a good way to get some solid conditioning without really thinking of it as a conditioning run. Also on racedays I always get a 20-30 minute AM run with a few strides. During my race warmups, after some experimentation, I found that I perform better after an intense (moreso than the normal warm up) warm up-I go 10 minutes real easy, 5 at AC to Marathon pace, then I am hitting half marathon to 10k pace the last 5-8 mins. Then throw in some drills and strides, get around 10 minutes then hit a few solid accels and then the gun goes. If I were to treat TT days like race day, it would get very close to 7 miles before cooldown of the second run. Again this is gained from trial and error, and by no means would work for everyone, but 15 miles would be rediculously low especially for upperclassmen who, IMO, should be given an progressive annual volume from freshman year all the way til they conclude their career.