Again, I would try the cheapest solutions first: I think as
madpogue mentioned with a lot of lever travel and little braking the cable housing could be compressing a lot; and no reasonable amount of adjustment of the yoke cable is going to compensate for rock hard brake pads. I would start with brake pads (you need good ones anyway), follow that with cable replacement. Both are wear items. But again based on my experience I had a similar situation with my cantilever brakes and it all worked out.
Another thought for a *really* cheap test: take the pads you have and grind off 0.5mm or so with a file - rough them up. If your braking improves even temporarily then new pads should be on the list. Of course they might be so hard all the way through that this makes no difference.
Also for reference: the pad at the top is a brand new one for my cantilever brake: its hard as a rock; the one below is a used pad for a V brake - also very common. When I changed out the top style for the bottom I also saw quite an improvement. I didn't think it would help and wanted to stay with the "original pads" but I am very happy I went with the bottom style - and they are much more available where I am located so that is a bonus too. Only thing is you may need to juggle spacers or add spacers to make things fit perfectly on your own bike.
EDIT: I just noticed you have the post style pads on your brakes - they make the bottom type of pad with the post style as well, and my comment about spacers is irrelevant as you can easily adjust things in and out without spacers.....