Cardio training gone wrong & right:
Whatever type of cardio training you choose (walking/running/cycling/swimming/rowing etc) it's important to measure to some degree what you're achieving so that you can look back to see what you have done right or wrong in your journey whether it be sport specific or for general health/fat loss.
A lot of people measure time when it comes to cardio. After all, the government recommendations are '30 minutes of cardiovascular training 3x/week' so it's no wonder we time our journeys to measure our success. However this can negatively affect our goals and judgment on what's right for us because one person can bike for 30 min and only ride 3km while another person bikes for the same amount of time and rides 7km.
Chances are no one that is reading this post is old and brittle and is actually capable of logging a few kilometers on their next run/bike etc.
My advice is to stop accepting 'time' as the number one determinator if you are making progress and instead measure your distance and use that as your mark. After you have narrowed down your achievable distance then hold yourself accountable to improve your time or improve your distance in time.
If you're doing tabata or HIIT style of cardio, measure your heart rate and recovery in the specific time frame.
Time in distance or time in recovery is where it's at! Don't rely just on time!