The word "tapas" comes from the Sanskrit verb "tap" which is "to burn". It's the burning of that which does not serve you and turn it into something useful. Like burning coal to make a train go.
Our modern world often feeds us messages like "no pain, no gain" but is this applicable to Yoga? We have to remember that the state of Yoga is the state of unity, harmony. Where there's no conflict between what the mind, body and soul wants.
So what if you wake up one morning and your body is sore? Should you still do a million Sun Salutations? What if your child has a high fever, should you still sit in meditation for an hour?
If your body is aching, go for a gentler practice. If you cannot meditate because of your child, meditate with/on your child. Tapas is not about creating inner conflict. It's quite the opposite. It's about finding peace, union and harmony in each and every moment as it passes. Choosing to remain in a state of joy, peacefulness and equanimity even when externally it doesn't seem like an option.
Tapas is more about consistency than quantity. Tapas is about finding your Sadhna (regular practice) and sticking to it. But it is also about living your Yoga. It is about choosing love over fear, choosing happiness over sadness, choosing forgiveness over hatred, choosing peace over conflict, choosing compassion over judgement, choosing health over illness, choosing Yoga against Ego.
Tapas is the willingness to start over even when you had "failed" before. Starting over with an open mind and innocently expectant heart. Starting over with hope and burning enthusiasm. Beautifully put by a dear Yogi, Tapas is "making hard decisions with a soft heart".