Won't an easier way will be to copy a large text from net, paste it into MS Word. Export each page as image. And then train it using that image and the known text.
It won't help that much, as every character/letter/digit will be of the same font face, e.g. Arial. Then it can only recognize scans with Arial font.
One needs a real world training set, a lot of variation, like house numbers and scanned books with labeled datasets.
I'm planning on adding a JSON config file to be able to tweak the network and add a lot more variation to the generated training set. Meanwhile, if you want to train it with several different fonts you can just add them to the 'fonts' array in captcha.js.