Ha, DtF, you talk sense too... you're a qualified coach ? (hoping to do some later this year)
No need to taper... maybe a mini taper, 1-3 days rest, whatever works for you. And you don't want to go flat out 100% effort anyway and then need too much recovery time or DOMs - likely a 16-19 mile long run the week after.
Tom/ Scott - Don't worry about negative splits, it very much depends on the course... If you can stick to a steady pace (pre-decided) and hold back a bit, get more out second half then fine. If its hilly, then negative split may be not possible.
Last year I did a weeks taper for a half, just light running .. didn't really gain anything from it - although I was a bit fresher after. Since then I usually stick to my normal weekly routine (Tue speed/Wed gym strength & conditioning, Thu tempo) before any races and just go lighter effort on the Thursday run to leave something in the tank.. If you're not 100% sure go easier earlier that week.
Nothing wrong with doing a warm up before, 13.1 race then 5-6 miles cool down either to get a 20 miler in. It will be slow and definitely in the right pace area by the end.
And what Susan said, where is WT?