Penance means being full of tenderness and kindness towards the suffering

Here is a recipe for your way as a Christian: pray, do penance, work without rest, fulfilling your duty lovingly. (The Forge, 65)

Penance is fulfilling exactly the timetable you have fixed for yourself, even though your body resists or your mind tries to avoid it by dreaming up useless fantasies. Penance is getting up on time and also not leaving for later, without any real reason, that particular job that you find harder or most difficult to do.

Penance is knowing how to reconcile your duties to God, to others and to yourself, by making demands on yourself so that you find enough time for each of your tasks. You are practising penance when you lovingly keep to your schedule of prayer, despite feeling worn out, listless or cold. (Friends of God, 138)

(http://www.opusdei.org.in/sec.php?s=310)