Paul

I would not do 20m as my last long run with a 2wk taper afterwards - you will not recover.

Your last few long runs could be 16,17, 20 & 18 and then into a 3wk taper.

If you keep up the intensity and reduce the mileage by 25% in wk 1, then by another 30% in wk 2 you will still be doing a fair few miles without the wear & tear.

At least 2 of the longer runs could be off road to further reduce the stress on your body - so if you can find a canal towpath close by that would be ideal

Good luck

Personally I would want to run more than 18 miles before a marathon.  I think a lot of plans go up to at least 20 miles, if not 21/22.  If you've already done a couple of 16 milers why not do another 16, then a couple of 18's and change the final two to 20's?  A plan is only there as a guide and generally I think it's best to adapt them at times to what you feel comfortable with.  if you think you'll feel better prepared by having done more than 18 miles, then I would do it...