Heh, it wasn't as simple as I had envisioned. But, I gave it a whirl. Because it took several slides and might get longer, I put it in a PDF. It's accessible here:
http://www.hkclark.com/Group Delay vs Sub Distance.pdf
I stopped short of trying to explain why group delay is undesirable (IMHO). But, that could be done in a couple of additional slides.
Let me know if there are any questions.