Many of them choose the bicycle deliberately, just to have at least a small amount of exercise before and after [the] hours they spend sitting at their desks.

deliberately 是副词，来自形容词 deliberate（故意的、蓄意的），表示"故意地、有意地"。 the 是定冠词，在这里特指"他们坐在办公桌前的那段时间"。