[Howard Klepper]

Wonder, I think what you didn't get is that there are four bass notes to the measure. On the 2 and 4 beats where there is a chord, the bottom note is one of the bass notes. Where the 4 beat is just a bass note without a chord, it is the leading tone to the next root.

...................................G.............. .......C.........................D
So the bass line is going 1 5 5 5/ 1 5 5 3/ 1 3 5 3/ 1 3 1 #1/ I etc.

As others have said going back to C instead of G on the 3rd beat of the 4th measure is just because it sounds better--it gives a stronger movement up to the D.

You will get confused thinking of leading tones going from the 7th to the root of the next chord, because a leading tone does not need to be a chord tone (if the chord is dominant or minor it won't be), and does not need to be below the root to which it leads. Leading tones are always a semitone below or above the root of the next chord.

A walking bass line, like an alternating bass line, is typically a quarter note on each beat in 4/4 time. But a walking line is not just root and fifth--it arpeggiates the chord to include root, third, fifth, and sometimes other chord tones, and often--but not always--uses leading tones to get to the next root. A walking line gives a feeling of forward harmonic movement, and is commonly associated with a swing rhythm.