It's supposed that surfaces greater than 3 feet wide are any floor, and tge base DC for Acrobatics is 0. However, if that floor is covered with gravel (slightly obstructed), the DC is +2. Such a surface (gravel) is difficult terrain and the movement on them is halved. Moving at full speed on uneven surfaces is DC +5. So, it is DC 7 to move at full speed on gravel. BUT no skill check is needed to move across such surfaces unless the DC is 10 or greater. That means: you can always move on gravel without penalties on your movement. Am I OK?
Thanks!