The way you phrased that kind of makes it sound like just another commitment to be avoided. Wouldn't you need to do something more like "give your friend $1000 in escrow and tell her she should only give it back to you if you run the marathon"?
This depends how trustworthy you are in general, and is definitely an interesting question.
Would Gandhi take a pill which made him 1% less likely to not commit any murders, if it meant receiving a million dollars with no other strings attached?
The answer seems to be, "he would use a Schelling Fence" if that was an option.
(tl;dr: they don't agree that announcing your goals makes you less likely to accomplish them. Which should not be surprising, to anyone who has already heard of Beeminder.)
You should read about commitment devices in general but the idea is they FORCE you to do something you know you're probably not going to do.
A good example is to tell your friend you're going to run a marathon and if you don't you'll give her $1000.