Después de caminar tanto, me dolían la cintura y el talón derecho.

Because después de is the fixed expression for after when it is followed by a noun or an infinitive.

• después de comer = after eating
• después del trabajo = after work

So here:

• después de caminar tanto = after walking so much

If you want a full clause with a conjugated verb, you usually use después de que:

• Después de que caminamos tanto...

But when the subject is the same and English would often use after + -ing, Spanish normally uses después de + infinitive.

Here tanto means so much or so long / so far depending on how you interpret the walking.

So:

• caminar tanto = to walk so much / to walk so far

It adds the idea that the amount of walking was a lot, which explains the pain afterward.

A very natural alternative is:

• Después de tanto caminar...

That means essentially the same thing, just with a slightly different emphasis.

The verb doler works a lot like gustar.

You do not usually say that the person is the grammatical subject. Instead, the thing that hurts is the subject, and the person affected is shown with an indirect object pronoun.

So:

• Me dolía la cabeza = My head hurt / My head was hurting
• literally: The head was hurting to me

In your sentence, the things that hurt are:

• la cintura
• el talón derecho

That is why Spanish says:

• me dolían la cintura y el talón derecho

Using estaba doliendo is possible in some contexts, but it is much less natural here. Spanish normally uses the simple past tense forms of doler, especially the imperfect, for this kind of ongoing physical discomfort.

Because the subject is plural: la cintura y el talón derecho.

Even though each noun is singular by itself, together they form a compound subject, so the verb must be plural:

• La cintura me dolía.
• El talón derecho me dolía.
• La cintura y el talón derecho me dolían.

So dolían agrees with the two body parts that were hurting.

The me tells you who was affected by the pain: me = to me.

With doler, Spanish normally uses these pronouns:

• me = to me
• te = to you
• le = to him/her/you (formal)
• nos = to us
• os = to you all
• les = to them/you all

Examples:

• Me duele el pie. = My foot hurts.
• Te duelen las piernas. = Your legs hurt.
• Le dolía la espalda. = His/Her back hurt.

So in your sentence, me does the job that English often expresses with my or with word order.

With body parts, Spanish very often uses the definite article (el, la, los, las) instead of a possessive like mi or tu, especially when it is already clear whose body part it is from the pronoun.

So:

• Me duele la cabeza. = My head hurts.
• Me lavé las manos. = I washed my hands.
• Me dolían la cintura y el talón derecho. = My waist and right heel hurt.

Using mi is not impossible, but it is less natural in a neutral sentence like this because me already tells us whose body parts they are.

La cintura usually means the waist.

In real-life usage, it can sometimes refer a bit more broadly to the waist area, depending on context. But the core meaning is waist, not hip and not exactly lower back.

So if someone means specifically lower back, Spanish often prefers something like:

• la parte baja de la espalda
• la zona lumbar

But in your sentence, la cintura is understandable as the waist area hurting after a lot of walking.

Because derecho is an adjective here, meaning right as in the right side of the body, and in Spanish most adjectives normally come after the noun.

So:

• el talón derecho = the right heel
• la mano derecha = the right hand
• el ojo derecho = the right eye

Here derecho does not mean straight. It means right-side.

Dolían is the imperfect tense, which is often used for:

• ongoing states in the past
• background description
• pain or discomfort that lasted for a period of time

So:

• me dolían suggests the pain was continuing or persistent

By contrast:

• me dolieron uses the preterite and sounds more like the pain happened as a completed event, or that you are viewing it as a whole.

In this sentence, the imperfect fits very naturally because it describes how you felt after all that walking, not a single sudden moment.

Yes, absolutely. That is very natural.

Compare:

• Después de caminar tanto...
• Después de tanto caminar...

Both mean roughly After walking so much.

The difference is small:

• caminar tanto puts the focus a bit more on walking a lot
• tanto caminar turns it more into so much walking

Both are correct and idiomatic.

It is very natural and recommended here because Después de caminar tanto is an introductory phrase.

So:

• Después de caminar tanto, me dolían la cintura y el talón derecho.

The comma helps the sentence read more clearly. In informal writing, some people might omit it, but with this kind of opening phrase, using the comma is a good standard choice.

In standard Spanish, no: with la cintura y el talón derecho as a compound subject, the verb should be plural:

• me dolían la cintura y el talón derecho

If you use singular dolía, it sounds grammatically mismatched because there are two things hurting.

If you wanted singular, you would need to change the structure, for example:

• Me dolía la cintura, y también el talón derecho.

But with both nouns directly linked as the subject, dolían is the standard form.