I learned J in very much this way, although instead of reading J code I chose to write it. I worked my way through a cheatsheet (http://www.jsoftware.com/jwiki/HenryRich?action=AttachFile&d...) inventing little toy problems to solve with particular verbs along the way. But words (verbs, gerunds, etc.) are not enough - you need to know the rules for composing them. Fortunately this is rather easy to learn.

I'd say that you need to know about hooks, forks (trains in general) and ranks before starting to learn the vocabulary. But maybe you can learn those things along the vocabulary too.