I think that starting the slide right after playing D will produce the singing effect that seems to be the purpose of the exercise.  So the slide is the means to this end. That's how it works when I play the exercise. If I delay the slide there is no effect of vocalism, just a kind of smeared F that shoots up from below.  I also found that starting out with the metronome at 100 as indicated was  a bit too fast for me for the first attempt. 100 makes sense because if it is much slower the slide will sound like a series of half steps. But for learning I started a bit slower  I also found it helpful in learning, to begin the first two measures at full speed with just the thumb strokes. C D, D- F. That helped me get the timing.

Thanks Jody, that's helpful. Great idea on isolating the thumb to begin with. I will try that.