You are getting things confused with a normal job, that looks after you. i.e 1700 you finish of what you are doing then go home, then come backt it the following mornong.

With Enterprise they get lots of business by promising a car to compnaies within 2 hours. Senior management have not taken into consideration whether there are enough cars or that some branches have an area of over 2 hours drive.
So what happens corporate business comes over for 0600 the following morning. Basically you cannot turn the deal down (GM are arseholes about that!!) and so you cannot go home untill all the deals are done.
So that is why Enterprise staff will work so late, because in most branches in is around 1500 on a wednesday 3 cars outside however 7 more deals to do for 0600 Thursday morning. All your local branches have no cars either, so you area manager will force your manager to force you to work till 2200 as you need to drive to west london to collect cars and then deliver them out as well and then wait to be picked up as you are probably part of a big run.

For the love of God, please re consider, you will hear lots of things like "its not a sprint, its a marathon" but in the mean time by the time you have realised to leave enterprise, all your friends will have sprinted past you, to bigger and better jobs.

My advise (which I am sure you have had lots of from different people) is to just wait for a while, and phone some agencies in the main areas. ie, london, liverpool, Manchester nearest to where you live. There are so many jobs out there that will offer a better future, with a better starting wage. And with Agency's they will sell you and your C.V.