The problem is a matter of wording. The core rulebook equates distance (walk rate, run rate) with terms that describe speed (walking, running, sprinting). For example, say we have a Human with an Agility of 5. Her walk rate would be 10 meters, and her run rate would be 20 meters. If this character wanted to "run" or "sprint" 8 meters away, would it be permissible to let that character apply the "running modifiers" or "sprinting modifiers" for that Phase?
SR5, p. 161, "Movement":
Walk rate determines the farthest a character can move during a Combat Turn before they are considered to be Running.
SR5, p. 162, "Standard Movement (Walking and Running)":
As soon as the character exceeds their Walk rate, they are considered Running until the end of the Combat Turn and incur any penalties or benefits of running.
The next sentence goes on to say that running consumes your Free Action, preventing you from doing a bunch of simple-seeming things (there's a table on the same page). The bottom of the column and over to the next page breaks out the other running modifiers.
The example on p. 162 is helpful. Nowhere does it indicate that Wombat can choose whether or not he's running or walking, even though logically one does not casually saunter to cover - you want to absolutely minimize your exposure when facing a hail of bullets. He has a Walk Rate of
Y and moves distance
X - he is either Walking (
X≤Y) or Running (
However, there is nothing I could find in the movement rules that require the path to be a straight line or in the same direction. Using the game's example, and the lack of anything prohibiting it, it is permissible and cinematically appropriate for somebody to back up
Z meters (cinematically psyching himself up to rush through that potential hail of bullets), then run forward to cover. If
X+(2*Z)>Y he's Running. (It's
2*Z because he has to regain the
Z meters he backed up.)
As far as Sprinting goes (SR5, p. 162), it isn't really a different type of movement than running, it just eats up actions to press your Run Rate higher. If you haven't exceeded your Walk Rate, you can make all the Sprint tests that paragraph allows - they just don't mean anything until you've reached your natural Run Rate.