Sounds like you have an extract only + hops recipe with no specialty grains.
In this case you are boiling the hops to extract the bittering compounds which your recipe needs. You need to do a 60 minute boil, but add the extract 45 minutes into the boil, with 15 minutes remaining. This ensures a full 60 minute boil, and gives your extract 15 minutes of boil time to kill off anything that may be in it.
Also, the reason you do the last extract addition is to preserve the colour and flavour which works a little better as a late addition rather than a whole boil. You could always boil half of the extract from the start, and add the rest as a 15 minute late addition.