You can delete extrudes by selecting all of the faces that have been extruded and clicking the DEL button on your keyboard.
Once you've deleted the extrudes, do as said above and delete the edges to turn it into one face
Now extrude this single face outwards, scale to your likings, modify certain areas, and repeat until your model is done.
<EDIT>
Here's a pic to help, basically it shows extruding one face from a cube, selecting all of the faces, deleteingthe extrusion and then deleting the original faces to make one big face.