Me duele el talón desde ayer, así que hoy no voy a correr.

Because doler works like gustar.

In this structure, the thing that hurts is the grammatical subject, and the person affected is shown with an indirect object pronoun:

• me duele el talón = my heel hurts / the heel hurts me

So:

• me = to me
• duele = hurts
• el talón = the heel

You can say mi talón me duele, but it sounds more marked or emphatic. The most natural everyday wording is me duele el talón.

Because the verb agrees with el talón, which is singular.

• Me duele el talón = My heel hurts
• Me duelen los talones = My heels hurt

So the rule is:

• singular thing hurting → duele
• plural things hurting → duelen

Spanish very often uses the definite article with body parts when the person is already clear from the pronoun.

So instead of saying:

• mi talón

Spanish often prefers:

• me duele el talón

The me already tells you whose heel it is, so el sounds more natural than mi here.

This is very common with body parts:

• Me duele la cabeza
• Me lavé las manos
• Le rompió la nariz

Talón means heel.

In this sentence it refers to the body part, the back underside of the foot.

Be aware that talón can also mean other kinds of heel or a similar part in other contexts, but here it clearly means the body part.

Because Spanish often uses the present tense to describe something that started in the past and is still continuing now.

So:

• Me duele el talón desde ayer = My heel has been hurting since yesterday

Literally, it looks like My heel hurts since yesterday, but in natural English we usually use has been hurting.

This is very common in Spanish:

• Vivo aquí desde 2020 = I have lived / have been living here since 2020
• Estoy cansado desde esta mañana = I’ve been tired since this morning

Usually, me duele el talón desde ayer is the more natural choice if the pain is still continuing now.

Using the present tense with desde often expresses an action or state that began in the past and continues into the present.

• Me duele el talón desde ayer = it started yesterday and still hurts now

Me ha dolido el talón desde ayer is possible in some contexts, but it is less straightforward for this ongoing meaning and may sound less natural than the present tense here.

Así que means so, therefore, or as a result.

It connects the first idea to the consequence:

• Me duele el talón desde ayer, así que hoy no voy a correr.
• My heel has been hurting since yesterday, so I’m not going to run today.

Other similar connectors are:

• por eso = that’s why
• entonces = so / then

But así que is a very natural choice here.

Because ir a + infinitive is a very common way to talk about a future plan or intention.

• hoy no voy a correr = today I’m not going to run

This sounds very natural in everyday Spanish.

You could also say:

• hoy no correré

That is correct, but it sounds a bit more formal, more definite, or simply less conversational in many everyday situations.

It is part of the future construction ir a + infinitive.

Structure:

• voy = I go
• a = linking preposition
• correr = to run

Together:

• voy a correr = I am going to run

This is one of the most common future expressions in Spanish:

• Voy a comer
• Vamos a salir
• No va a llover

Yes, you could, and it would be understandable.

• Hoy no corro = I’m not running today
• Hoy no voy a correr = I’m not going to run today

The second one makes the idea of intention or plan a little clearer. In this sentence, because the speaker is deciding not to do something due to the pain, no voy a correr sounds especially natural.

Because me is the indirect object pronoun that shows who is affected by the pain.

The basic pattern is:

• me duele... = it hurts me...
• te duele... = it hurts you...
• le duele... = it hurts him/her/you(formal)...
• nos duele... = it hurts us...

The pronoun usually comes before the conjugated verb:

• Me duele la espalda
• Te duele la pierna
• Nos duelen los pies

Yes, but it is usually omitted unless you want emphasis or contrast.

• Hoy no voy a correr = normal, natural
• Yo hoy no voy a correr = more emphatic, like I’m not the one who’s going to run today

Spanish often leaves out subject pronouns because the verb ending already shows the person:

• voy already tells us it is I

The word order is very natural and standard:

• Me duele el talón
• desde ayer
• así que
• hoy no voy a correr

A few points:

1. Me duele el talón is the normal order with doler.
2. Desde ayer comes after the main clause and tells you when the pain started.
3. Así que introduces the consequence.
4. Hoy is placed before no voy a correr to highlight today.

You could move some parts around for emphasis, but the original sentence sounds completely natural.