How can I stitch together several m2ts video files into one large file?
I believe you can join MPEG ts (transport stream) files simply by joining them together.  In Linux:

cat file1.m2ts file2.m2ts file3.m2ts &gt; joined_file.m2ts


In Windows/DOS:

copy /b file1.m2ts + file2.m2ts + file3.m2ts joined_file.m2ts /b


As long as the input files are split properly, and each new file begins with a key frame (and I would expect your camera does this), this should work just fine. And even if your camera is not careful about splitting right before a keyframe (again, I'd be surprised if it doesn't), as long as you re-join in the exact same order, it ought to work.