The ornamental design for a runner's straw, as shown and described.
FIG. 1 is a rear perspective view of a runner's straw showing my new design, wherein a partial view of a hand is shown in broken lines for illustrative purposes only and forming a part of the claimed design;
FIG. 2 is a front perspective view of a runner's straw showing my new design, wherein a partial view of a hand is shown in broken lines for illustrative purposes only and forming a part of the claimed design;