the onward journey must occur by the 60th day. However you can apply for a 30 day extension for 1900 THB at a local immigration office. As a US passport holder, you are visa exempt for 30 days. To extend for another 30, you can leave the country or visit immigration for the extension. The new 60 day visa you are referring to is not a visa, but a visa waiver for 60 days. This has not been implemented yet. I am not sure when it will be. There was wild speculation that it would be implemented on June 1. Now I have heard July 1, September, etc. It will be official when we read it in The Royal Gazette. I am not sure when you are coming, but you could apply online through a Thai Embassy website for the 60 day visa. Again, you may be asked for proof of onward travel when checking in for your flight. You could get a cheap ticket from Bangkok to say, KL, Vientiane, etc. Some ppl will read this and may say they never been asked for proof and that is true. Also google “onward travel” for further information. You will get a list of sites that you will find interesting.