What you really need for headwinds is this:
Which, as per the description, can go directly into the wind faster than the wind itself, powered by the wind. Although it's maybe a little large and unwieldy to be of much use on the towpath.
If you want to waste an evening, google about for the story behind that cart, also look for the downwind faster than the wind version, as when it was proposed a few years ago a great number of people said it was impossible to do so the guy ended up building the thing to prove otherwise. There is a physics based explanation of why something so unintuitive is possible though it took me a long time and several headaches to understand it.
Sorry, I'll stop blowing this thread off topic now....