How to refer to a city (or town) that is not on the coast?

How could one convey the idea that a place is “in the middle” of the geographic area of a country, thus far from the seaside — whether city or town, rural or urban?

The fact that it’s a city or town or rural or urban is not relevant. The only relevant information would be: far from the sea; non-coastal.

This word (or phrase, expression) would be used in sentences like this: “I don’t go to the beach very often, I live [...]” — without needing to say one’s exact location or explain too much, if possible.

Does such straightforward expression (preferably a single word) exist?

In American English, a city which is inland from the sea can be referred to simply as an inland city. The AHD definition—

adj. 1. Of, relating to, or located in the interior part of a country or region: inland freshwater lakes and ponds.

—is typical, but I would say that inland as used in AmE does not usually mean "toward the middle," but rather "away from the coast," and may be very relative depending on context. The Los Angeles neighborhood of Brentwood is inland from Pacific Palisades, but that isn't much an impediment for getting to the beach, even in Los Angeles traffic.

I hesitate to speak for British English as I know inland is frequently a synonym for domestic (e.g. inland post, Inland Revenue), and geographic sensibilities as to direction and distance are very different, so too with Australia.

If you want to specify that a city is very deep inland, you could refer to it as an interior city—

adj. 1. Of, relating to, or located on the inside; inner. 3. Situated away from a coast or border; inland.

[AHD]. While

Interior more strongly indicates something close to the middle and far from the coasts; Sacramento or Albany or even Washington, D.C. are inland, but I would not expect them to be called interior cities, even if by a technical definition that would be true. Salt Lake City is an interior city, St. Louis is in the interior. In the U.S., this sense may be reinforced because the U.S. Department of the Interior is the division of the federal government which deals with federal lands, natural resources, and Native Americans—far away (or driven away) from the major cities, and again I cannot speak for elsewhere, where an interior ministry is something completely different that has no real U.S. equivalent.

Distance from the borders with Canada or Mexico is more nebulous a requirement; I wouldn't refer to Detroit or El Paso as interior cities, but others might.

Another approach, as @Billy suggests, is to describe the city based on geographic terms that indicate distance from the sea. If I describe Lincoln, Nebraska as a Great Plains city, or Helena, Montana as nestled in the Rockies, or even that Louisville is in Kentucky, someone familiar with American geography will know that they cannot be situated anywhere near the ocean. This only works for a handful of regions, however.

The further danger is that there are cultural associations with certain geographic terminology that may not make sense, particularly with ill-defined terms like heartland or Middle America. Boulder, Colorado is in the middle of America, but it is not of Middle America; its stereotypical resident would not be labeled a Middle American, but its stereotypical resident would not mind that in the slightest.

As to the setting of a city in the countryside, I don't understand your meaning. The very definition of city means that wherever the city is located is not rural. You will encounter farms and ranches and open space driving an hour outside Kansas City, Missouri, but you can also find farms and ranches and open space an hour outside San Diego or Philadelphia.

"I live in the city"? - Does this automatically convey the idea of "no sea"?

No, it does not automatically convey the idea of "no sea." That's because "city" doesn't convey anything about seas, not that it's on a sea, not that it's not on a sea.

Moscow isn't on a sea. Paris isn't on a sea. Madrid isn't on a sea. New Delhi isn't on a sea. Munich isn't on a sea. Prague isn't on a sea. Mexico City isn't on a sea. These are all huge world cities and none of them are on the sea, so what makes you think the word "city" has anything to do with being on a sea?

Terms you might use to describe a city that isn't on a sea:

• Omaha is in the heartland of the United States.
• Las Vegas is landlocked, even having no rivers or navigable waterways.
• Kansas City is in the interior of the United States, almost a thousand miles from any shore.
• Indianapolis is smack dab in the middle of middle America.
• Cheyenne, its state's capital and largest city, is in rural Wyoming.

Non-coastal (city / place / area / state), according to Oxford Living Dictionary:

(of an area) not near or having a coast.

One example sentence by ODO:

‘There are some places, notably Shetland, where the art of using fish livers has been developed by a fishing community to an extent which is unimaginable for persons in non-coastal urban environments.’

In your example:

"I don't go to the beach very often, I live in a non-coastal city.

The benefit of this answers over the others is that is straightforward. Most people know what you mean right away.

It's also defined by Eurostat, a Directorate-General of the European Commission (emphasis is mine):

Coastal areas are defined on the basis of the local administrative units or municipalities (LAU-2) and consist of those municipalities (or equivalent local administrative units) within a given country or NUTS 3 region that are bordering the sea or close to the sea:

*if a municipality borders the sea, it is by default coastal;

*if a municipality is not bordering the sea but has 50% of its surface within a distance of 10km from the sea, it is also considered coastal;

All other municipalities are non-coastal.

Attribution: "Non-coastal | Definition of Non-coastal in English by Oxford Dictionaries." Oxford Dictionaries | English. Accessed July 16, 2018. https://en.oxforddictionaries.com/definition/non-coastal.

Eurostat. "Glossary:Coastal Area." - Statistics Explained. Accessed July 16, 2018. http://ec.europa.eu/eurostat/statistics-explained/index.php/Glossary:Coastal_area.