"Непаханое поле" – a [big] amount of undone work.
Updated example: a kid is leisurely watching TV while there a lot of undone homework (which he hasn't even started).
Note: the example below is probably misleading. The thing that something is overlooked, the lack of information is an artifact of the example, not inherent to "untilled field" Russian expression.
For example, a person does some finalizing, "polishing" bits of a work (or just lazying about) not realizing that actually the scope is bigger and he should be doing the intensive, more real thing instead.
Imagine someone removing snow from some ground who keeps cleaning up little bits of remaining snow, not knowing that he has missed a chunk of ground with "untilled" snow.
Best Answer
The idiom a long row to hoe fits pretty well (given the clarification), and it also fits with the original's agricultural theme. Usage:
or