Learn Hardcore Portuguese (Portugal): A maratona parece impossível, mas ela tem ganho força todos os dias. - The marathon seems impossible, but she has been gaining strength every day.

With specific things: A maratona de Lisboa é famosa. – The Lisbon marathon is famous.
With general concepts too, much more than in English: A vida é difícil. – Life is hard.

So A maratona parece impossível is the natural way to say The marathon seems impossible.

Why is maratona feminine, and why do we use a and ela for it?

In Portuguese, all nouns have grammatical gender (masculine or feminine), including things and concepts.

maratona is feminine → so we use:
- the feminine article a: a maratona
- the feminine pronoun ela: mas ela tem ganho força…

Even though a marathon in English is it, in Portuguese it behaves like she because of grammatical gender. Examples:

O problema é grave, mas ele tem solução.
The problem is serious, but it (he) has a solution. → problema is masculine, so ele.
A cidade é antiga, mas ela é muito bonita.
The city is old, but it (she) is very beautiful. → cidade is feminine, so ela.

So ela here can mean:

she (a woman already mentioned in the context), or
it (referring back to a maratona, which is grammatically feminine).

Could we leave out ela and just say mas tem ganho força todos os dias?

Yes, you can. Portuguese is a pro‑drop language: subject pronouns (eu, tu, ele, ela…) are often omitted when the subject is clear.

A maratona parece impossível, mas tem ganho força todos os dias.

This is perfectly natural if it’s obvious from context what is “gaining strength”. You keep ela when:

you want to avoid ambiguity, or
you want to add emphasis to the subject:

…mas ela tem ganho força todos os dias.
…but *she has been gaining strength every day (not someone else).*

What tense is tem ganho and what does it mean exactly?

tem ganho is the pretérito perfeito composto, formed with:

present of ter
- past participle
  - ter → tenho, tens, tem, temos, têm
  - past participle of ganhar → ganho

So:

ela tem ganho força ≈ she has been gaining strength / she has gained strength repeatedly

In European Portuguese, this tense usually describes:

an action that started in the past,
has been repeated or ongoing,
and is still relevant now.

Compare:

Ela ganhou força.
She gained strength (at some point in the past, finished event).
Ela tem ganho força.
She has been gaining strength (over a period, little by little, and still is).

What is the difference between tem ganho força and está a ganhar força?

Both can be translated as has been gaining/is gaining strength, but they’re not identical.

está a ganhar força (present progressive, European style):
- focus on what is happening right now or around now
- more “in progress” feeling
tem ganho força (pretérito perfeito composto):
- focus on an action repeated or continuing over a period up to now
- suggests a series of gains, a process over time

Examples:

Ela está a ganhar força.
You’re looking at her right now and noticing she is gaining strength.
Ela tem ganho força todos os dias.
Over many days, again and again, she has been gaining strength.

In your sentence, with todos os dias, tem ganho fits very well because it suggests a repeated, ongoing improvement.

How is tem ganho different from English has gained?

English has gained is ambiguous:

She has gained strength (recently, result now), or
She has been gaining strength (over a period).

Portuguese makes a stronger distinction:

Ela ganhou força.
→ a completed event in the past (she gained strength).
Ela tem ganho força.
→ repeated or ongoing process up to now (she has been gaining strength repeatedly / little by little).

So tem ganho is closer to has been gaining than to a simple has gained.

Why do we use parece impossível and not é impossível?

The verb changes the meaning:

A maratona é impossível.
The marathon is impossible. → a firm statement: it is impossible.
A maratona parece impossível.
The marathon seems/looks impossible. → it appears impossible, from our point of view; maybe it isn’t actually impossible.

parece softens the statement; it shows perception or impression, not an objective fact.

Could I say A maratona parece ser impossível? Is there any difference?

Yes, you can say:

A maratona parece ser impossível.

This is grammatically correct and understandable. The nuance:

parece impossível is shorter, more natural, and is what people would usually say.
parece ser impossível sounds a bit more formal or careful, like:
- seems to be impossible (slightly more analytical or “distanced” style).

In everyday speech, parece impossível is the default choice.

Why is impossível the same with a maratona (feminine) – shouldn’t it change form?

Some adjectives in Portuguese do not change form for gender; they are invariable in that sense. impossível is one of them.

um desafio impossível – an impossible challenge (masc.)
uma maratona impossível – an impossible marathon (fem.)
desafios impossíveis / maratonas impossíveis – plural

It only changes for number (singular/plural), not for gender.

Other common adjectives that don’t change for gender:

difícil – difficult
fácil – easy
feliz – happy

What exactly does força mean here? Is it only physical strength?

força is a very flexible word. It can mean:

Physical strength
- Ele tem muita força. – He is very strong.
Endurance / stamina / fitness
- In a training context, ganhar força can mean improving physical condition.
Mental strength / determination / motivation
- Ela está a ganhar força can mean she’s becoming more determined or confident.
Momentum / support / influence
- A ideia tem ganho força – The idea has been gaining traction/support.

So depending on context, in your sentence it could be:

the runner’s physical strength,
her mental resilience,
or even the “marathon project” gaining support.

Why is it todos os dias and not just todos dias?

In Portuguese, when todo(s)/toda(s) is followed by a countable noun like dia, you normally use the definite article:

todo o dia – the whole day / all day
toda a semana – the whole week
todos os dias – every day
todas as semanas – every week

So:

todos os dias = every day (each individual day)
todo o dia = all day (long)

Omitting the article (todos dias) is not correct in standard Portuguese.

Can tem ganho força also mean “has been gaining popularity” or “has been becoming more important”?

Yes. ganhar força is often metaphorical and very common in the sense of:

to gather strength / gain traction / grow in importance

Examples:

A proposta tem ganho força no Parlamento.
The proposal has been gaining ground in Parliament.
A ideia do teletrabalho tem ganho força desde 2020.
The idea of remote work has been gaining strength since 2020.

So your sentence can easily be understood in that figurative way as well, depending on context.

How would this sentence sound in natural spoken European Portuguese? Is it already natural?

The sentence is already natural and correct. Depending on context, a speaker from Portugal might also say:

A maratona parece impossível, mas ela tem ganho muita força todos os dias.
(adding muita for emphasis)
If referring clearly to a person already known, they might drop the noun:
- Parece impossível, mas ela tem ganho força todos os dias.

But structurally and stylistically, your original sentence:

A maratona parece impossível, mas ela tem ganho força todos os dias.

is fully natural in European Portuguese.