I am still breastfeeding my 21 month old son and am just completing the weaning process...last week at once a day.
I stopped pumping months ago and have had no trouble keeping up the supply. My body has adjusted to the new decreased demand, and so has my son.
I am pregnant again, which is why I am stopping. I've slowly weaned him over the past 3 months from 4 times per day (morning, after work, before bed, middle night) to just before bed. My son was very distraught when I tried to wean him earlier, and despite public scrutiny, I decided to continue as I know that it is beneficial well beyond the 12 months that is recommended.
Good luck with the extended breastfeeding. Your body is made for this and will adjust to the changes.
For more information on extended breastfeeding, go to www.mothering.com.