Do sign up and march if you can. It will make a difference.
The US has been a primary source for the quicksand prior climate talks have encountered. They are slow realize their effect on the climate, and slower to make real commitments to changing their contribution to greenhouse gas emission.
That said, the tone is changing, and US government officials are listening. The thing is that making a decision that something has to be done is quite a distance from making a decision to do something. And that is some distance away from actually doing. The US appears to be somewhere between those first two steps.
That is why the head count at this march is so important. Climate is not a kooky fringe issue anymore. Everyone needs water. The folks in Washington understand that...