After trying the HD render numerous times and Windows 7 64bit REPORTING the length as 1hr 1and 1 min I did a 30 min test render. Windows reported it was 26 min, but per a suggestion from "aquaholik" I used a STOP WATCH and the movie was actually 30 min long. In Windows Media Player it hit the 26 min mark and played for another 4 minutes!
So, as the video was playing, Windows Media Player showed "26.xx as the length? And did that time stay frozen at 26.xx even as the cursor hit the end and kept playing? Curious.
I just built a Win-7 64-bit system, and on the first video I rendered (MPEG-2) and played from my hard disk, Windows Media Player had a couple "glitches" where it got a momentary freeze (1/4 second or so) -- and this exact glitch was repeatable at the same place, in multiple renders, and on a scene with very low motion. I downloaded another player (VLC, I think) and the video played fine.
I'm guessing there are issues with Windows Med Player -- maybe related to 64-bit? I think I've heard folks on the Vegas Pro forum complain about problems with the current version of WMP.
It happens lot of time with .VOB files too, not just .m2v or .m2ts. It gets to the "reported length" and keeps playing until it really ends at the actual length.