Мне пора бы чаще делать кардио, а не только упражнения на пресс.

Because пора is used in an impersonal construction.

The pattern is:

кому + пора + infinitive

So:

• Мне пора идти = It is time for me to go
• Тебе пора спать = It is time for you to sleep

Here, мне is in the dative case, showing the person whom the situation applies to. Russian does not say this with a normal subject like я пора.

Пора by itself means something like it is time.

Adding бы makes it softer and more reflective. In English, it often feels like:

• I really should...
• It is probably time for me to...
• It is about time I...

So Мне пора бы чаще делать кардио sounds like the speaker is thinking to themself, with a mild sense of self-criticism or postponed obligation.

Compare:

• Мне пора делать кардио = It is time for me to do cardio
• Мне пора бы делать кардио = I really should be doing cardio / It is about time I did cardio