When I try to run indoors on a treadmill, my caves burn like no other. When I run outside it seems that I am just fine. But since it is still cold where I live and icy, I tried using an elliptical. I generally stay on level 12, and I am able to keep my heart rate in the 160s for a half hour (I am 22 years old). However, when I use the elliptical I barely break a sweat. I've tried turning up the resistance but it just makes my legs burn. I feel that if my heart rate is in the 160s I'm doing something right, even though it seems easy. Should I be doing a much more strenuous workout? I'm sort of confused as to whether or not what I'm doing is too easy.