Is there an idiom like the Russian's "Untilled field"?

Solution 1:

The idiom a long row to hoe fits pretty well (given the clarification), and it also fits with the original's agricultural theme. Usage:

That's a long row to hoe.

or

He has a long row to hoe.

Solution 2:

Russian idiom непаханое поле can have slightly different meanings, depending on a on a context, but in general means that there is many work that can/need to be done.

As a russian speaking person I feel that best match would be:

have a lot/enough on your plate

Usage:

Don't you have enough on your plate?

Which is equivalent to russian:

У тебя же дел непаханое поле?

In a context that you have too much work to do already/barely started to do that you already have.

It can also be combined with:

Business before pleasure

Which is equivalent to russian Делу время - потехе час.

For example:

Business before pleasure, you have a lot on your plate.

Делу время - потехе час, у тебя работы непаханное поле.

Solution 3:

There is a well known expression: Rearranging the deckchairs on the Titanic.

Focussing on small irrelevant details when something much larger is going on all around.
It does carry a connotation of impending disaster if the larger event is not addressed, that may not be what you are after.