Translation:I exercise twice a week.
In this context (doing physical movment) "exercise" and "work out" are often used interchangeably. 运动 has to do with motion while 锻炼 has to do with strengthening/ training, so "work out" is probably best covered by 锻炼 but that doesn't mean it shouldn't be accepted here.
So what is the difference between "运动" and "锻炼" ? Both have meant exercise. I understand "锻炼" is also to work out, but it also meant exercise.
Is the first word a noun and the second one a verb? Why don't we need to place a "的" before the activity. Is it not necessary if we use "次" ?