Workout

I signed up for cross-fit course. It is high intensity training using a variety of different exercise disciplines including weight lifting, cardio and flexibility. It is tougher than I expected, and my muscle pain has lasted for a long time, but I am gonna enjoy suffering to build my muscle. I think learning languages could be applied in a same way  as building muscles. It takes time, and sometimes it goes with pain and suffering. I will achieve my goals no matter how hard it is. I am really happy to imagine my changing body.

[Good work.  Your grammar and vocabulary were correct for the most part.  I just changed some phrases to make the sound more natural.]

Workout
I signed up for a cross-fit course. It is high intensity training using a variety of different exercise disciplines including weight lifting, cardio, and flexibility. It is tougher than my expected, and my muscle pain lasts for a long time after the exercise, but I am gonna enjoy suffering to build muscle. I think learning languages could be thought of in the same way as building muscles. It takes time, and sometimes it comes with pain and suffering. I will achieve my goals no matter how hard it is. I am really happy to imagine my changing body.

Your writing is quite good, some suggestions:
"cross-fit course" needs an article in front of it ('a' if there are several courses like it or 'the' if you a referring to a specific course and there is only one course that meets your definition - ex: I signed up for the cross-fit course offered on Mondays at my university)
"Build muscle" is a common expression, "build my muscles" is also correct but "build muscle" is more natural sounding