If you want to become good at a particular subject.activity, If you want to become good at a particular activity,

Subject could be correct, but running is more of an activity. Also I’d join this sentence with the next one.

If you want to be good at a particular subject., (add the next sentence) If you want to be good at a particular subject, (add the next sentence)

This is a dependent clause and cannot stand on its own.

If you want to be good at a particular subject. If you want to be good at a particular subject.

Tthe only wayfactor is how much time you can put oin. the only factor is how much time you can put in.

Or “the only way is to put in a lot of time.”

The only way is how much time you can put on(previous sentence), the only way is by putting in time. (previous sentence), the only way is by putting in time.

The only wayquestion is how much time you can put oncan you spend on it. The only question is how much time can you spend on it.

FThe first time when I managed to complete a full marathon course., The first time I managed to complete a full marathon course,

FThe first time whenthat I managed to complete a full marathon course., The first time that I managed to complete a full marathon course,

This is a dependent clause.

FThe first time when I managed to complete a full marathon course. The first time when I managed to complete a full marathon course.

I usedspent a whole year prepareing for it. I spent a whole year preparing for it.

Again, these two sentences need to be linked.

I used(previous sentence), I spent a whole year prepareing for it. (previous sentence), I spent a whole year preparing for it.

I usedspent a whole year to prepare for it. I spent a whole year to prepare for it.

Each month I will joined at least one 10 mile or half marathon competition. Each month I joined at least one 10 mile or half marathon competition.

You’re talking about what you did in the past, when you were preparing for your first marathon, so this needs to be in past tense.

Each month I will joparticipated in at least one 10 mile or half marathon competition. Each month I participated in at least one 10 mile or half marathon competition.

Past tense

Each month I willould join at least one 10 mile or half marathon competition. Each month I would join at least one 10 mile or half marathon competition.

Each week I will at least practicetrained for at least 6 hours. Each week I trained for at least 6 hours.

I’m not a runner so I’m not sure, but “practice” is more about improving technique, while “training” could be for improving endurance and stuff.

Each week I will at least practicepracticed at least 6 hours. Each week I practiced at least 6 hours.

Past tense

Each week I will at least practiceould practice at least 6 hours. Each week I would practice at least 6 hours.

AtIn the end I did completed a full marathon course. In the end I completed a full marathon course.

Or “ In the end I did complete a full marathon course.” (This way emphasises that it really did happen).

At the end I did completed a full marathon course. At the end I completed a full marathon course.

No need for "did".

At the end I did completed a full marathon course. At the end I did complete a full marathon course.

Actually, the result was not even close to “good”a high standard. Actually, the result was not even close to a high standard.

Good doesn't work here, and no reason for quotations.

Actually, the result was not even close to “good” standard. Actually, the result was not even close to “good” standard.

But, without any mentoring, just pure “will” power that was a fair outcome. But, without any mentoring, just pure will power that was a fair outcome.

No need for quotes, will power is a standard phrase.

But, without any mentoring, just pure “will” power, that was a fairgood outcome. But, without mentoring, just pure willpower, that was a good outcome.

But, without any mentoring, just pure “will” power that was a fair outcome. But, without any mentoring, just pure willpower that was a fair outcome.

GA good amount of time, and good amount of “brain” power is the key to success. A good amount of time and good amount of brain power is the key to success.

Good amount of time, good amount of “ and brain” power is the key to success. Good amount of time and brain power is the key to success.

No reason for quotations.

GA good amount of time, and a good amount of “brain” power is the key to success. A good amount of time and a good amount of brain power is the key to success.