I'd echo several of the above posts by saying that a Goenka retreat is no more (or, necessarily, less) a marathon than a retreat of a week or more in the style of other Theravada teachers (such as Mahasi Sayadaw/U Pandita, Pa Auk Sayadaw, etc, etc). To me the Goenka retreat I sat a few years ago was quite standard Theravada. Goenka has his particular interpretation of the Suttas (as do all teachers), teaches a particular style of meditation (as do all teachers), and has a particular chanting style. Some could quibble with some technical details of interpretation (as they probably could with all teachers), but as I recall, the Dhamma talks were rather standard 4NT, 8FoldPath, etc.
Hmm, that sentence got rather complex. The point is that in my view the differences between Goenka and other Dhamma teachers is no wider than the general differences between Theravada teachers.