Yesterday afternoon, after work, I rode my bike to go to my mom's home.

"rode" already implies that you went somewhere, so you don't need "to go" as well.

When I arrived at that bun store, I bought ten yuan buns.

This is a bit ambiguous. Ten buns that cost 1 yuan each? This is probably cultural context as much as translation, but I'd maybe say "ten one-yuan buns".

If it was in the old days, such a distance was a piece of cake, but now I feel a little tired.

You need a conditional tense with "if" here. "If it was in the old days, such a distance [would be/would have been] a piece of cake". But "if" isn't really necessary, it's more natural to just talk about what it was like in the old days.

You'd be more likely to use "if" here to speak hypothetically about a very specific situation. "If you'd offered me that price ten years ago, I'd have bitten your arm off."

I was no exception, in that several bikes of mine were stolen.

Or just "I was no exception, and several ..."

Technically incorrect to use "I was no exception" here, it means "I belonged to the same set as the previous thing I mentioned", but the previous thing you mentioned was "many thieves who stole bicycles". So it kind of suggests that you, also, were a thief who stole bicycles. It would be more natural if you'd said "Many people were targeted by bicycle theft, and I was no exception" - but it's the kind of error natives make all the time and only English teachers and copy editors care about, so in a sense not really an error at all. Here it's quite natural, and talking about your bikes being stolen makes it perfectly clear what you mean. But using "exception" in the same way in another context might produce worse results.

Nowadays in China, there are very few people who still ride bikes because each family almostalmost every family has a car.

"each family almost has a car" - every family has *most of* a car. Like, they might be missing one wheel, or a window, or the wing mirror. But they all have one. Or they're all very close to getting one, they "almost have a car" because they're buying one next week.

Not really a problem here because it's clear what you mean. But "each of my friends almost passed the exam" and "almost all of my friends passed the exam" - very different meanings.

People who ride bikes nowadays aredo so not because they can't afford a car, but because they just want to practice their body to get moreexercise and get healthyier.

Difficult to explain the grammar here, but what you had was wrong. "do so", "do it", "do that", not "are". It would be "are" only as "People are riding bikes nowadays not because ...".

You probably wanted "train", not "practice". "practice" doesn't really work here. I suggest "exercise" because it means you can drop the "body", and then I don't have to worry about whether it should be "bodies" to match the plural "people" :)

I know this journal is written messily, but I don't want to do thisit's not intentional.

Need the adverb here, "messy" is an adjective.

"I don't want to do this" is like someone is forcing you to write the journal.