This is a method i came up with which I have found to be very useful if you want to zone into Old hillsbrad but cant get past mobs of zf to get past the invisible walls =)
1.Zone into old hillsbrad
2.Close WoW
3.Take out the file from your WoW data folder
4.Open WoW, you should be falling under zf, quickly alt tab then right click WoW and close
5.Put the file back in
6.Open WoW
7.You should now be under Old hillsbrad, swim past the walls and then keep jumping till u get to land ( Be careful you dont swim too far or you'll fall off the edge of the world =) )
8.Explore!!! =)
I found this method out myself =)
For those who are to lazy or incapable of doing it by getting deep in zf T_T