Front ac vents in a 2003 Ford Expedition

When I start the truck the front vents blow cold air, when the truck reaches running temp and I start to drive it blows extremely hot air, feels like it is coming straight from the motor. Go faster, hot air comes in faster, go slower is comes in slower. But it is constant with or without the ac on. I drove 4 hours today and ended up with burns on my foot because of the floor vents. The rear ac works great all the time, but the front has to be on for the back to be on. So I suffer to keep the kids comfortable. Any ideas on what the problem might be? And how do I fix it? I have am 8 hour drive in 2 days with an already burned foot.