I have triedam trying to exercisinge as often as I can to stay healthy, but it isn't easy to do because of COVID-19. I am trying to exercise as often as I can to stay healthy, but it isn't easy to do because of COVID-19.

I have tried exercising as often as I can, in order to stay healthy. I have tried exercising as often as I can, in order to stay healthy.

Your sentence is perfect. This correction is purely stylistic.

I have triedtry to exercisinge as often as I can to stay healthy. I try to exercise as often as I can to stay healthy.

I presume you are writing in the present tense so it would be "I try"

I have tried exercisinge as often as I can to stay healthy. I exercise as often as I can to stay healthy.

I have tried to exercisinge as often as I can to stay healthy. I have tried to exercise as often as I can to stay healthy.

But it isn't easy to do because of COVID-19. But it isn't easy to do because of COVID-19.

연결사인 "but"으로 문장을 시작하는 게 좀 어색한 거 같아요 (문법적으로 틀린 거 아닌데 구어적인 표현에 가까운 거 같아요)!

Before the pandemic, I went to the gym after work, and sometimes I went to the swimming pool. Before the pandemic, I went to the gym after work, and sometimes I went to the swimming pool.

Before the pandemic, I went to the gym after work, and sometimes I went to the swimming pool. Before the pandemic, I went to the gym after work, and sometimes I went to the swimming pool.

Again, a purely stylistic suggestion.

Before the pandemic, I went to the gym after work, and sometimes I went to the swimming pool. Before the pandemic, I went to the gym after work, and sometimes I went to the swimming pool.

Before the pandemic, I went to the gym after work, sometimes I went to the swimming pool. Before the pandemic, I went to the gym after work, sometimes I went to the pool.

"Swimming pool" wouldn't be wrong, but you can just say "pool"

Before the pandemic, I usually went to the gym after work, and sometimes I went to the swimming pool. Before the pandemic, I usually went to the gym after work, and sometimes I went to the swimming pool.

I also walked around my office building orand climbed the stairs of my house. I also walked around my office building and climbed the stairs of my house.

WhenDuring the holidays, I used to ride my bicycle over thanand cycle over 60 km. During the holidays, I used to ride my bicycle and cycle over 60 km.

WhenDuring the holidays, I used to ride my bicycle over than 60 km. During the holidays, I used to ride my bicycle over 60km.

"During" is essential. You can choose either, "ride my bicycle more than 60km." or, more naturally, "ride my bicycle over 60km."

WhenDuring the holidays, I used to ride my bicycle over than 60 km. During the holidays, I used to ride my bicycle over 60km.

When there were holidays, I used towould ride my bicycle over than 60 km. When there were holidays, I would ride my bicycle over than 60 km.

WhenDuring the holidays, I used to ride my bicycle overmore than 60 km. During the holidays, I used to ride my bicycle more than 60 km.

Except climbing the stairs,The only thing I can't do other things nownow is climb the stairs. The only thing I can do now is climb the stairs.

Except climbing the stairs, I can't do other thingsanything else now. Except climbing the stairs, I can't do anything else now.

Except for climbing the stairs, I can't do the other things now. Except for climbing the stairs, I can't do the other things now.

ExceptAside from climbing the stairs, I can't do those other things now. Aside from climbing the stairs, I can't do those other things now.