I ran into that problem and fixed it by Odin-ing back to stock and starting over. Sorry.
However, you might be able to just get an older version of TWRP and just flash that over Odin.
Edit: In reply to Phateless.
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In case anyone else does this to themselves (derp), I solved the problem. It was easy, actually.
I just redownloaded the zipfile for the modified TWRP, pulled the recovery.img file out of the zip, and reflashed it with Heimdall.