First, you will want to use "the" before the word "gym". The word "the" in English is a definite article with many uses, in this case, you will use it because "the gym" is a specific place-- it is the gym you go to after work. It also helps specify that you go there frequently. "The" is also often used to refer to everyday things (nouns) that most people are familiar with. (For example, "I always take the train.") The article "the" has a lot of uses, but I hope my explanation helped. Next, in the underlined sections, "to do exercises" is not incorrect, but it can be simplified by replacing it with the phrase "exercising". For example, the sentence "I don't like to do exercises" can be changed to "I don't like exercising". For your second example, "I do regular exercises", it is more natural to say "I exercise regularly". "Regular" is an adjective, and adding -ly like in "regularly" makes it an adverb. An adverb modifies a verb, adjective, or another adverb. Adverbs can be confusing and is a more difficult part of English grammar, so don't feel bad! For the sentence "I do office work, so I hardly move on working", you will want to use "while" in place of "on". "While" is used to describe two things happening at the same time-- you are working, and at the same time, you are sitting down/not moving much. For the sentence, "I almost sit", you have a few options, but "I usually sit" or "I often sit", sounds the most natural. Both sentences describe that most of the time that you are at work, you are sitting down.

Exercises are specific training procedures that you follow ; you can do exercises for your heart and circulation - aerobic exercise; you can do muscle strengthening exercises, etc. Exercise is the general term that includes different exercises. A particular exercise or several exercises are countable [there is singular and plural], exercise in general is uncountable.