Many athletes have the approach to training that the more you do, the better it is. This is a truth with modifications. Most swimmers know the term “garbage yardage”, which is where you’re just going through the motions, and not really putting in a solid effort either technically or intensity wise.
I am a firm believer in quality in training, and that you can go very far with a focused effort even with limited time available. Most people have a lot of demand on their time (from family, work, social life etc.), so there is only so much time for training. So why not get the most out of it? I am not saying that you should just go full throttle every time, but that you have to be better after each session than you were before. Focus on efficiency, and getting the most out of the time that is put in.
This can be done by working on a technical detail, or doing a new personal best up that hill. However, it can also be that you are relieving some of the stress that has built up after a tough day at work, and thereby giving yourself new energy.
Set a goal before each training session, make it realistic, and then go for it. But if you have a bad day, and you can’t do the 3 miles at race pace you wanted to do, then let it go, and make sure you get something else out of the day.
Always finish your training sessions with something good technically (a perfect 25 yard freestyle, running with good posture, etc.), as your muscle memory is then allowing you the best chance of starting the next session out on a good note.