Long runs are necessary because they help to increase the number of mitochondria in your aerobic cells. That means you'll get more ATP production. Long runs will greatly increase your fitness. The 3200 and 1600 are still aerobic events. You should be doing speed work, V02 max work, and long runs each week during racing season if those are your two events.
Also, something recommended by Jack Daniels - long runs should be kept at a maximum of 25% weekly mileage. So if you run a 50 mile week, your long run should be no longer than 12ish miles. Of course, this is subject to debate, but it is a general rule that is good to adhere to so that you don't get injured by running too far in one go.