It will hugely depend on your life pre children and support network post children of how much changes for you afterwards.
If you are a bit of a homebird and family orientated e.g. spend time with nieces/nephews and friends children, have 2 sets of useful and local grandparents nearby, are well off and a supportive partner you will have freedom and may not need major adjustment.
If you don't have the above it will be a lot harder. Particularly if you life involved things like exotic travel, love partying, high flying career where long hours, flexibility and extra commitments are expected, you like a high level of control, spontanity and childfree friends family or hobby that requires time.
Like others have said, even if you get time, you may be too tired to.take an opportuity e.g. an exercise class when you have had 4 hours broken sleep that night.
I'm always surprised there are not higher levels of post natal mental health issues when you think of the huge change of lifestyle combined with sleep deprivation. It's comparable to experiences things like a serious illness/bereavement/redundancy in some ways. Your body is changed, you can lose your identity, there is a new family/friend/partner dynamic, your earning potential drops.