1. (answering your PM) Checking your ip through the command prompt will never give you the actual ip the internet sees you at, so what you just described is normal.
2. If you want to connect to yourself, use 127.0.0.1 (thats how I connect to my server).
3. Pinging 68.70.87.172 shows that the ip is reachable. So that means there's absolutely nothing wrong between the rest of the internet, and your modem/router. However, from the modem/router to your computer, there's a problem.
4. Have you forwarded your ports? Most people who try to host a Terraria game, and have probems, usually don't forward their ports. If you want to forward your ports, use the following site:
www.portforward.com
Find your router, find Terraria, follow instructions. You'll probably need to also give your computer a static IP address (its actually not necessary, but it can save yourself a lot of headaches if you do give your computer one).