Time training or reps and sets?

I just had a question about working out- becuase I just satrting working out just a few days ago and i am already unmotvated. I am currently training to be a distance runner, and I am building up my muscular endurance by doing 30 minutes of quad-workout thingymabobers non-stop every night. Am I training the right way? Should I be doing the repetition/set format of working out instead of my 30 minute non-stop workoput routine? The only real reason why I ask this is becuase I am really unmotivated even after a few days, becuase every time I workout, it just means pain without really being able to set and reach goals for myself. Thanks for your time.