If your goal is fitness only, I would think that uneven ground and ups and downs would be better than flat ground.
“Many men go fishing all of their lives without knowing that it is not fish they are after.” - Henry David Thoreau
"Life is a daring adventure or it is nothing" - Helen Keller
"Keep not standing fixed and rooted, briskly venture, briskly roam" - Johann Wolfgang von Goethe
"Wenn ist das Nunstruck git und Slotermeyer? Ja! Beiherhund das Oder die Flipperwaldt gersput!"