We claim the ornamental design for an exercise cycle, as shown and described.
FIG. 1 is a front, side perspective view of an exercise cycle;
FIG. 2 is a side view of the exercise cycle of FIG. 1;
FIG. 3 is a top view of the exercise cycle of FIG. 1;
FIG. 4 is a front view of the exercise cycle of FIG. 1; and,
FIG. 5 is a rear view of the exercise cycle of FIG. 1.
The ornamental design that is claimed is shown in solid lines in the drawings. The broken (dashed) lines shown in the drawings are included for the purpose of illustrating environmental elements only and form no part of the claimed design.