That belief comes from our competitive, patriarchal (sorry guys), on-the-way-out interpretation of the world. But it is only a belief, an idea, the idea that we must struggle to overcome and win, struggle to be worthy of whatever, battle uphill to make the progress we want. Pain and soreness and lots of sweat, in this belief system, are proof that we've worked hard and deserve a stronger body, more abs, more muscles, more endurance, more stamina.
When I think of traditional Asian practices like qi gong or yoga, though not the Westernized competitized yoga versions, you are taught to listen to your body, to honor it. These practices are gentle on the body, yet very effective in removing energy blockages, and strengthening body and mind gradually over time. The instructions are specifically to work with your body instead of forcing it into something that hurts or that the muscles are not yet ready for.
It's a fundamentally different approach, one of respecting and loving your body, of connecting and cooperating with it, of tuning into it. If you've had an injury, honor that, and be gentle in that area. Your muscles are stiffer in the morning, acknowledge that when you work out early. It's not because your workout buddy is able to do something that you have to compete with her and prove that you can too. Ballroom dancing, by the way, is great exercise. It is cooperative, relaxing, non-competitive, social, and a great right-brain left-brain balancer. Very beneficial for both body and mind.