Hi sahuaro. I see you posted on 5/3 and I'm hoping maybe I can help you a little bit. I've been to Chichen Itza near Cancun. We went by way of bus in June and almost died of heat exhaustion. It was a long journey. There is also Tulum. Another far distance. Both of these near Cancun.
Have you looked at The San Gervasio ruins? Check out www.tripadvisor.com
for information on these ruins. They are in Cozumel. As for the distance, I'm not sure. Cozumel just isn't that big of an area. I am positive that the 5 hours of time needed for Caba Myana is inclusive of travel/time spent at the ruins, and possibly lunch.
As far as the excursion being "worth it", it totally depends on how badly you want to see it. I knew 20 years ago that I wanted to see Chichen Itza. It was amazing to me.
If you are not the snorkeling or diving type, then go for it. Otherwise, I would say to do something water related. Cozumel is known for it's beautiful diving water and reefs. It's your call. But, check out the website first because it sounds like you really want to see the ruins. Have fun and let me know how you make out.
BTW... welcome aboard.