Learn Russian: Ты добегаешься, твоё дыхание уже очень тяжёлое. - You will run to exhaustion, your breathing is already very heavy.

This prefix and postfix pattern creates a perfective verb indicating that an action is done to a negative extreme or limit. Built from бегать (to run), добегаться means to run until you exhaust yourself or bring about a bad consequence. It is frequently used as a warning.

Why is добегаешься translated as future tense if it looks like a present-tense conjugation?

Adding the до- and -ся affixes for a negative outcome makes the verb perfective. Because perfective verbs cannot have a present tense, conjugating them with present-tense endings always creates the future tense. It emphasizes that this bad result will definitely happen if the action continues.

Is добегаться only used for literal physical running to exhaustion?

In this sentence it describes literal exhaustion from running, but this pattern is heavily used for figurative warnings. You will often hear similar verbs like допрыгаешься (from jumping) or доиграешься (from playing) used as idioms meaning that you are going to get yourself into serious trouble.

Why use тяжёлое with дыхание here?

Just like in English, the adjective тяжёлый (heavy) is used to describe labored or difficult breathing (тяжёлое дыхание). At a B2 level, you will see тяжёлый applied to many non-physical things, such as a severe illness, a difficult feeling, or a hard situation.