When I was a kid living in North Iowa I only fished with "nightcrawlers," big ones. I don't remember them ever being brittle.
They are like rubber, therefore, hooking them was no problem.
From what I recall, the only time they break apart is if you purposely "pull" on them, thereby stretching them to break.
The skinny Texas worms, now that another story. I've never used them, but I'd assume they stretch as well.
I guess I cannot help you with regards to "toughening" them up. I'd say to refrigerate them, but that actually might make them more brittle.