Others have brought up some good points.
Depending on the field, you might need to be flexible to move just about anywhere, and possibly more than once.
Don't assume that some institution of your choosing will be planning to hire in your area in a given year.
You will likely be better motivated than most and can realistically get out in under 4 years.
Do not do this if you think that a PhD is some ultimate credential.
Once you complete that it is just the beginning. Every idiot around you will have one. You have to pick up your effort even further at each subsequent stage (postdoc, asst prof, etc).