Хотя день и был длинным, я всё равно рад, что купил абонемент и сделал хотя бы короткое кардио.

This is a very common Russian pattern for concession:

Хотя X и Y, ... = Although X, ...

So here:

Хотя день и был длинным...
means Although the day was long...

The и does not mean a separate and here. It reinforces the concessive idea and is often used after хотя. You can think of it as making the contrast sound more natural or emphatic.

You could also say:

Хотя день был длинным, ...

That is also correct, but Хотя ... и ... is a very standard pattern.

Because after быть in the past tense, Russian often uses the adjective in the instrumental case when it functions as the predicate:

день был длинным = the day was long

So:

• день = nominative subject
• длинным = instrumental predicate adjective

This is very common:

• Он был уставшим. = He was tired.
• Фильм был интересным. = The movie was interesting.

You may sometimes hear nominative adjectives after быть, especially in more colloquial or stylistically different speech, but был длинным is standard and natural.

Yes, you could.

Both длинный and долгий can be used with time-related things, but there is a nuance:

• долгий often focuses more directly on duration
• длинный can sound a bit more subjective, like the day felt long

So:

• день был долгим = the day lasted a long time / was long
• день был длинным = the day felt long, was a long one

In everyday speech, both are possible here. The sentence you were given is understandable and natural.

Here всё равно means all the same, anyway, or even so.

So:

Хотя день и был длинным, я всё равно рад...
= Although the day was long, I’m still glad...

It marks contrast: the long day could have been a reason not to feel good about it, but despite that, the speaker is glad.

Important: всё равно can also mean I don’t care / it makes no difference, depending on context. But here it clearly means still / nevertheless / anyway.

Рад is the short form of the adjective рад = glad.

Russian often uses short-form adjectives for temporary states or feelings:

• Я рад. = I’m glad.
• Я готов. = I’m ready.
• Я уверен. = I’m sure.

Because the speaker is masculine here, it is рад.

If the speaker were female, it would be:

• Я рада, что купила абонемент и сделала хотя бы короткое кардио.

Notice that several words change:

• рад → рада
• купил → купила
• сделал → сделала

In Russian past tense, verbs agree with the gender and number of the subject.

Since the subject is я, the verb still shows gender in the past tense:

• я купил, я сделал = male speaker
• я купила, я сделала = female speaker

So the sentence tells you the speaker is male.

This is normal for Russian past tense:

• Я был дома. = I was at home.
• Я была дома. = I was at home.

English does not show this, but Russian does.

They are perfective because the speaker is glad about completed results:

• купил абонемент = bought the pass/membership
• сделал кардио = did/completed a cardio session

Perfective is very natural here because the speaker is referring to actions as finished accomplishments.

If you used imperfective:

• покупал
• делал

it would sound more like process, repetition, or background action, which does not fit as well with I’m glad that I did it.

So рад, что купил ... и сделал ... strongly suggests satisfaction about completed actions.

Because Russian commonly uses рад, что... to mean glad that...

So:

• Я рад, что пришёл. = I’m glad that I came.
• Я рад, что купил абонемент. = I’m glad that I bought a pass.

The что introduces a subordinate clause explaining what the speaker is glad about.

This is a very useful pattern:

• Я рад, что...
• Я счастлив, что...
• Я доволен, что...

Абонемент is usually some kind of pass, membership, or subscription giving repeated access.

In this sentence, because of the fitness context, it most likely means:

• a gym membership
• a fitness pass
• a pass for multiple visits

It is not just any general English subscription. In Russian, абонемент often refers to something that lets you attend repeatedly, such as:

• a gym
• a pool
• a course
• a concert series
• a library service in some contexts

So here, купил абонемент probably means bought a gym membership/pass.

Хотя бы means at least, if only, or even just depending on context.

Here:

сделал хотя бы короткое кардио
= did at least a short cardio workout
or
= did at least some short cardio

It suggests the speaker may not have done as much as they ideally wanted, but even a small amount still counts as something positive.

This phrase is very common:

• Скажи хотя бы слово. = Say at least a word.
• Подожди хотя бы минуту. = Wait at least a minute.
• Купи хотя бы воду. = Buy at least water.

Yes, кардио here is being used as an indeclinable noun meaning cardio workout / cardio session.

In modern spoken Russian, especially in fitness language, кардио is very common.

Because it is typically treated as neuter, the adjective is also neuter:

• короткое кардио
• интенсивное кардио
• утреннее кардио

So:

сделал хотя бы короткое кардио
means something like
did at least a short cardio workout

Even though кардио does not change its ending, the adjective still agrees with it in gender/number/case as neuter singular here.

Because сделал presents the cardio as a completed session.

In fitness-related Russian, сделать кардио is very natural and often means:

• to get a cardio session done
• to complete some cardio

It has the same feel as English I got my cardio in or I did a cardio session.

If you said делал кардио, it would emphasize the process more and could sound less result-focused. In this sentence, the speaker is happy that they actually completed it, so сделал fits better.

Russian uses commas to separate subordinate clauses very regularly.

There are two subordinate clauses here:

1. Хотя день и был длинным
2. что купил абонемент и сделал хотя бы короткое кардио

So the commas are:

• after the concessive clause:
  Хотя день и был длинным, ...
• before the что clause after рад:
  я всё равно рад, что...

This punctuation is standard and required in normal written Russian.