It’s normal to just run the distance that you want during each session. It’s no shame to run only 3km if you feels like it. No one know the body better than you, listen to your body. There are days where I setoff to complete a distance but cut it short. Likewise, there are days where I feel that I can go much further and just hit the distance, when my schedule allow.
We are not aiming for getting the correct distance for the day but completing the journey as a Marathoner, there’s another day to run.