If we have the right connection with our voice and these two aspects of expression (singing/speaking) we really don't feel much difference between the two.
If you want to know what I mean, just say a few words over and over in a comfortable speaking range and move around a bit up and down a comfortable range. Naynaynay works well. Then, at some point just hold one "nay" for about 10 seconds. This long "speaking note" will become a singing note. If it doesn't work the first time, try again until you figure it out.
Exercises using the speaking voice to strengthen the singing voice have been around forever. The currently popular voice training methods use the speaking voice to train good intonation, register blending and and discovering the natural breath support.