I did this a couple of years ago too, (probably 2010) having not done a huge amount of training! As Gladys says, the current isn't great, but that depends upon the amount of rainfall and how close to the bank you swim!
Certainly do-able, easilly, if you can swim 1.25km in a pool now! Again, as Gladys says, water quality isn't great, but it won't kill you! Just have a can of coke at the end to kill off anything you may have picked up.
My time was actually slower than when I did Outlaw last year!
But then I always got cramp in my calves in any swim above 2k wearing my old wet suit and this was no different! It's relatively easy to 'sight' on the bank if you only breathe on your left side!
The event isn't too serious so, even if you take a couple of hours, you'll enjoy it!