Is it normal for disc brakes to smell of burnt plastic?
Sounds like you may have "glazed" your pads and rotors. New brake pads require a break-in period before you subject them to hard long braking. 

http://www.bikeradar.com/us/gear/article/workshop-how-to-bed-in-disc-brake-pads-31337/

Even when you already have your disks broken in, long drag braking can cause them to overheat and fade. This leads to a kind of vicious cycle in which the brake is less effective and you start using it even more. 

Brakes are designed to cool themselves with the available airflow, even a few seconds every minute can make a difference. Of course, that's easier said than done when you feel like you are over your head. Braking effectively is a skill that takes practice. If you are dragging the brakes constantly to control your speed, you'll be better off coming to a complete stop and regrouping. 

Sometimes, you just need to walk stuff when you are over your head. 

http://www.singletracks.com/blog/mtb-training/how-to-brake-your-mountain-bike/