It turns out that that the thing you are supposed to be doing is the hardest to do. Here's my work log for the summer so far:
Grade Student papers / submit grades May 12
Reviewed article for [Academic Journal] May 21
Reviewed article for [Academic journal] June 10
Professor X's (at XXU) tenure letter June 12
Review of "Poesia y química" June 13
Finish conference talk June 14
You'll notice that I did all these things, but the "to do list with a single item" had something else on it, something that I did not do / have not done yet. I kept putting other, smaller tasks before the single item on my list. So maybe my idea of the list with a single item was not so brilliant after all. It works fine, as long as you don't want to actually do main thing you set out to do.