The ability to sing runs and riffs well depends on the flexibility of your voice, and not all singers have a flexible voice.
Practice runs and riffs by slowing the tempo and ensure that you are singing each note of the run/riff cleanly and distinctly. When you are sufficiently convinced that you are singing all the notes of a run/riff cleanly, then increase the tempo of the music until you are singing the runs/riffs at the intended tempo. Ask a friend to closely listen to your singing the runs and riffs for that friend's feedback on how well you are singing, or record your singing of the runs/riffs. Don't be discouraged if you learn that you have difficulty singing runs/riffs, because not all singers have flexible voices to sing runs/riffs.
I am a professional singer. For a run/riff I can sing four separate notes of the run/riff each second, but I don't have the flexibility of voice to sing more than four notes.