If you want to be good at a particular subject.

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

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

I usedspent a whole year to prepare for it.

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

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

At the end I did completed a full marathon course.

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

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

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

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.

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

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

This is a dependent clause.

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

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

Past tense

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

Past tense

At the end I did completed a full marathon course.

No need for "did".

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

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

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

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

No reason for quotations.

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

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

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

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

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

I usedspent a whole year prepareing for it.

Again, these two sentences need to be linked.

Each month I will 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 week I will at least practicetrained 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.

AtIn the end I did completed a full marathon course.

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

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

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

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