Quando eu corro demais, fico com dor no peito e no pescoço.

Because this sentence describes a habitual/general situation: Whenever I run too much, I get pain in my chest and neck.

In Portuguese, when quando refers to something that regularly happens, the present tense is normal:

• Quando eu corro demais, fico com dor...
• Quando ele estuda muito, fica cansado.

You would use the future subjunctive (quando eu correr) if you were talking about a future event:

• Quando eu correr amanhã, vou devagar.
  = When I run tomorrow, I’ll go slowly.

Yes, you absolutely could.

Portuguese often drops subject pronouns when the verb form already makes the subject clear:

• Quando eu corro demais, fico com dor...
• Quando corro demais, fico com dor...

Both are natural.

Including eu can add a little emphasis, clarity, or just reflect personal speaking style. In everyday Brazilian Portuguese, both versions are common.

Because correr is conjugated as:

• eu corro
• você/ele/ela corre
• nós corremos
• eles/elas correm

The rr stays in the stem. So:

• infinitive: correr
• eu corro

This is completely normal for this verb.

Also, pronunciation matters here:

• corro = from correr (I run)
• coro = choir or chorus

So the double rr helps preserve the correct sound.

Here, demais means too much or excessively.

So:

• corro demais = I run too much

It is similar to muito, but not identical.

Compare:

• corro muito = I run a lot
• corro demais = I run too much / excessively

So demais often suggests more than is good, healthy, or necessary.

In this sentence, that fits perfectly because it leads to a bad result: pain.

Both can work, but ficar com dor is very common when talking about ending up with pain or starting to feel pain.

Difference in feeling:

• fico com dor = I get pain / I end up with pain
• tenho dor = I have pain

In your sentence, fico com dor emphasizes the result of running too much.

Examples:

• Quando eu corro demais, fico com dor no peito.
• Depois do treino, fiquei com dor nas pernas.

This is a very natural structure in Brazilian Portuguese.

Dor usually means pain.

In this sentence:

• dor no peito = chest pain
• dor no pescoço = neck pain

Portuguese often uses dor + em/no/na + body part:

• dor de cabeça = headache
• dor nas costas = back pain
• dor no braço = arm pain

So dor is a very common and useful noun for physical pain.

Because Portuguese normally uses a preposition here: dor em + body part.

Then em + o contracts to no:

• em o peito → no peito
• em o pescoço → no pescoço

So:

• dor no peito = pain in the chest
• dor no pescoço = pain in the neck

This is one of the most common contraction patterns in Portuguese:

• em + o = no
• em + a = na
• em + os = nos
• em + as = nas

Portuguese uses definite articles with body parts much more often than English does.

So where English says:

• in my chest
• in my neck

Portuguese often says literally:

• no peito = in the chest
• no pescoço = in the neck

The possession is understood from the context. Since the subject is I, listeners naturally understand it means my chest and my neck.

This is very normal in Portuguese.

Because no peito e no pescoço both depend on dor.

So the structure is:

• fico com dor
• no peito
• e no pescoço

In other words:
I get pain in the chest and in the neck.

Portuguese, like English, often avoids repeating words unnecessarily.

You could repeat it for emphasis, but it would sound heavier:

• fico com dor no peito e com dor no pescoço

That is grammatical, but less natural here.

The comma separates the when-clause from the main clause:

• Quando eu corro demais, fico com dor...

This is similar to English:

• When I run too much, I get chest and neck pain.

In Portuguese, it is very common to put a comma after an introductory subordinate clause like quando...

Usually, yes, in everyday language.

Peito commonly means chest or breast area, depending on context. In a sentence about exercise and pain, dor no peito is naturally understood as chest pain.

If you wanted a more technical/anatomical word, you might hear tórax, but that is less everyday:

• dor no tórax = more clinical/medical
• dor no peito = normal everyday Portuguese

So peito is the most natural word here.

In most everyday contexts, yes.

Pescoço is the normal word for neck in Brazilian Portuguese.

Examples:

• dor no pescoço = neck pain
• machuquei o pescoço = I hurt my neck

So this is exactly the word a learner should know for ordinary conversation.

Yes, but the meaning changes slightly.

• Quando eu corro demais = When I run too much
• Quando eu corro muito = When I run a lot

Muito can simply mean a lot, without necessarily implying excess.
Demais more clearly suggests overdoing it.

Since the sentence describes a negative consequence, demais is especially appropriate.

Fico is the present indicative of ficar:

• eu fico
• você/ele/ela fica
• nós ficamos
• eles/elas ficam

In this sentence, the present tense expresses a repeated/typical result:

• Whenever I run too much, I get pain...

So it is not necessarily happening right now. It describes what generally happens.

A natural Brazilian pronunciation would sound roughly like:

KWAN-do eu KOH-ho de-MYSH, FEE-ko kon DOHR nu PAY-tu ee nu pes-KOH-su.

A few useful notes:

• quando: the qu sounds like kw
• corro: the rr in Brazilian Portuguese is usually a throaty h-like sound
• demais often sounds like de-maish in many Brazilian accents
• no is a contraction of em + o
• pescoço has stress on ço: pes-CO-ço

Of course, pronunciation varies by region, but this gives a good general Brazilian idea.

Yes. The original sentence is natural and everyday. A more formal or medical version could be something like:

• Quando corro em excesso, sinto dor no peito e no pescoço.
• Quando pratico corrida em excesso, sinto dor no tórax e no pescoço.

Tone differences:

• corro demais = everyday
• corro em excesso = more formal
• fico com dor = everyday
• sinto dor = a bit more direct/formal
• peito = everyday
• tórax = medical/technical

For normal conversation, the original sentence is excellent.