Learn Hardcore German: Mit jeder Trainingseinheit wird meine Ausdauer besser, und auch meine Haltung wirkt sicherer. - With every training session my stamina gets better, and my posture also appears more confident.

mit = with in the sense of accompanying circumstances
Here it expresses that every training session goes together with an improvement in endurance. You could paraphrase it as:
- Bei jeder Trainingseinheit (on/at every training session)
- Durch jede Trainingseinheit (through each training session)
The whole phrase functions as a time- or condition-setting element:
- Mit jeder Trainingseinheit wird meine Ausdauer besser.
  = Each time I train / every session I do, my endurance gets better.

Why does the sentence start with Mit jeder Trainingseinheit instead of Meine Ausdauer wird mit jeder Trainingseinheit besser?

Both orders are correct:

Mit jeder Trainingseinheit wird meine Ausdauer besser.
Meine Ausdauer wird mit jeder Trainingseinheit besser.

German main clauses follow the verb-second rule: the conjugated verb must be in second position.

If you start with Mit jeder Trainingseinheit (a prepositional phrase), that whole phrase counts as position 1, so the verb wird has to come next (position 2). The subject meine Ausdauer then comes after the verb.

Starting with the prepositional phrase simply emphasizes the circumstance (every training session) more strongly than the subject.

Why is it wird meine Ausdauer besser and not ist meine Ausdauer besser?

werden + adjective describes a change or development: something becomes or gets a certain quality.

wird besser = gets better / becomes better
ist besser = is better (a state, not the process of change)

The sentence talks about progress over time: with each training session, your endurance improves. That’s a change, so wird besser is the natural choice.

What exactly does Ausdauer mean here? Is it the same as "fitness"?

Ausdauer literally means endurance or stamina – the ability to keep going for a long time without getting tired.

It is not the same as general fitness:

Ausdauer = endurance/stamina (e.g. for running, cycling, cardio)
Kondition is close to Ausdauer, but can sometimes sound more like overall condition/cardio fitness
Fitness (in German) is a broader term, used like in English, but often associated with general physical condition, strength, flexibility, etc.

In this sentence, Ausdauer is specifically about lasting longer during training.

What does wirkt in meine Haltung wirkt sicherer mean? Is it "works"?

Here wirkt does not mean "works" (as in "functions").

In this context, wirken means to appear / to seem / to come across as to other people.

So:

Meine Haltung wirkt sicherer.
= My posture appears more confident.
= My stance looks more assured.

It describes how your posture is perceived, not necessarily how it objectively is.

What is the difference between wirkt sicherer and ist sicherer?

ist sicherer = is more secure/confident (a real, factual state)
wirkt sicherer = appears more secure/confident (how it looks to others)

So:

Meine Haltung ist sicherer.
→ My posture really is more stable or confident.
Meine Haltung wirkt sicherer.
→ My posture comes across as more confident; this focuses on outward impression.

The original sentence emphasizes how your posture looks as a result of training, which makes wirkt very natural.

Could I say …und auch meine Haltung wird sicherer instead of wirkt sicherer?

Yes, you could say:

Mit jeder Trainingseinheit wird meine Ausdauer besser, und auch meine Haltung wird sicherer.

This is grammatically correct and understandable. However:

wird sicherer says your posture actually becomes more secure.
wirkt sicherer says your posture appears more secure to others.

The original uses wirkt to focus on what people see: you look more confident.

Why is it sicherer and not mehr sicher?

German normally forms comparatives of adjectives with -er, not with mehr.

sicher → sicherer (more secure/confident)
lang → länger (longer)
stark → stärker (stronger)

mehr + adjective is only used in a few special cases:

with some adjectives that don’t form a regular comparative, or
to emphasize quantity rather than degree.

But sicherer is the standard comparative form of sicher, so mehr sicher would sound wrong here.

Is Trainingseinheit one word? How is it formed, and what does it literally mean?

Yes, Trainingseinheit is one compound noun.

It is made of:

das Training = the training/workout
die Einheit = unit

So literally it is a "training unit", which in natural English is:

a training session
a workout

Because it’s a noun in German, it is capitalized: Trainingseinheit. Its gender is feminine: die Trainingseinheit.

How does the word auch work in und auch meine Haltung wirkt sicherer? Could it go somewhere else?

auch means also / too / as well.

In und auch meine Haltung wirkt sicherer, the auch directly emphasizes meine Haltung:

…und auch meine Haltung wirkt sicherer.
= and my posture, too, appears more confident.

You can move auch, but the nuance shifts slightly:

Und meine Haltung wirkt auch sicherer.
→ and my posture also appears more confident.
(The "also" is more loosely connected to the whole clause.)
Und meine Haltung wirkt sicherer auch.
→ This sounds unnatural in standard German and is usually avoided.

The original und auch meine Haltung … puts the "also" right in front of meine Haltung, clearly marking posture as another item in the list of things that improve.

Why is the verb wirkt at the end of meine Haltung wirkt sicherer in the overall sentence?

The sentence actually has two main clauses:

Mit jeder Trainingseinheit wird meine Ausdauer besser,
und auch meine Haltung wirkt sicherer.

In each German main clause, the conjugated verb goes in second position:

Clause 1:
- Position 1: Mit jeder Trainingseinheit
- Position 2: wird
- Rest: meine Ausdauer besser
Clause 2:
- Position 1: auch meine Haltung
- Position 2: wirkt
- Rest: sicherer

So inside the second clause, wirkt is not at the end; it is in the second position, exactly where it belongs. The adjective sicherer naturally comes after the verb.