Im surprised nobody else asked this question, I went to surf on Route 10 and jus ended up back in oldale town. help.
You're surfing at the wrong place.
Start from Mauville city and go down until you reach the sign of the Cycling Road building (landmark). Keep going down until you reach a gap between the trees where you can surf.
Surf to the left until you reach the supports of the Cycling Road.
Go down a couple tiles, then continue going going left under the bridge.
Get off at the grassy area and you should be there.
Hopefully that helped. I also added a picture (which hopefully you can see), just excuse my inability to draw straight lines.
Hope this helps more and you have fun catching magnemite and voltorb or something. Just make sure you have the basement key.