That's maybe the longest period that I haven’t posted anything!

I got / I have / too many excuses not to write.

The best thing I can do is to start writing again.

TheIt is good thing about constant / regular writing or keep repeatingo be consistent and to write regularly.

I wasn't sure what you were trying to say here. So this correction is how I interpreted your original sentence.

It is like going to the gym and train up your muscle.working out.

"Training up your muscles" sounds a little weird, but not grammatically wrong.

Actually, I start a “short period but constaistent” exercise now.

For example I will do push ups within one to two hours.

I may start with 20-30 at first then I take 5 minutes rest / break / and start again.

As result I managed to do more than 200-300 push ups within a day.

Nice! That's a lot of push ups.

I never throught I canould do that.

Long time no writeing

That may be the longest period I haven’of me not posting anything!

Maybe is the wrong word to use here, I believe you meant "may be"

Lazy?

"Is it because I'm busy? Lazy?"

I gothave too many excuse nots to write ( them all ).

The best thing I can do is ( to ) start writing again.

The good thing about constant / regular writing or keep repeatingWriting regularly is like going to the gym and training your muscles.

Your original sentence is incomplete. I think you should join it with the next one like I did here.

Is like go to gym and train up your muscle.

Joined with previous sentences.

Actually, I start a “short period but constant” exercise nowhave started exercising in short bursts.

For example, I will do push -ups ( within one to two hours / in the timespan of one to two hours ).

I may start 20-30 at first then Ibegin with 20-30 reps, take a 5 minutes brestak and start again.

When we're doing an exercise, we call each movement a "rep" (short for repetition).

As result, I mcanage to do more than now do 200-300 push -up withins a day.

I never throught I canould do that.

Or:
"I never thought that could be possible."

That may be the longest periodtime I haven’t posted anything!

"May be" here is a form of the verb "to be" (you can confirm by replacing it with "is" or "might be"). "Maybe" is an adverb, so you need to be careful about the space.

"The longest time" is a bit of a set expression. If you wanted to use "period", it would need more connective words (for example, "That may be the longest period of me not posting anything").

Present perfect verbs use the past tense (have + v-ed, so "haven't post_ed_").

I got too many excuses not to write.

"Many" is used with countable nouns, so the noun must be in plural form.

The good thing about constant / regular writing or keepis the frequent repeatitiong.

You can't separate the subject from the verb in sentences like that.
I'm not sure how you intended to structure this part so I hope what I wrote is close to what you meant.

"Constant" mostly refers to something that's done non-stop, without taking a break, and sometimes with a bad connotation ("constant whining", "constant nagging").

It's like go toing to the gym andto train up your muscles.

This part ends a bit too abruptly. It feels like it's missing a description of how writing and going to the gym are related.
It would be best to add something like: "The more you do it, the better you get at it."

Actually, I recently started a “short period but constant” exercise nowworkout routine of doing the same exercise over and over in short intervals.

I wasn't too sure how to best rephrase this, so I hope this isn't too far from what you meant.

For example, I will dofocus on push up withins for one to two hours.

I may start at 20-30 reps at first, then I take a 5 minutes rest and startgo again.

"Reps" is short for "repetitions" and is a common workout term.

As a result I can manage to do more than 200-300 push up within as in one day.

"Within a day" to me has a connotation of being spread evenly throughout the day. I think this way is a little more common.
You could also say "in one session" to narrow down the time period even more.

I never throught I canwould be able to do that.

Be careful with "though/through/thought/tough".