I feel this way with every physical activity I do, and that's the primary reason I do what I do. I don't do it because it's exercise and it's healthy, screw that. I do it because this is what my body was made to do. I do it because the sensations feel so amazing, beyond anything I could ever describe with words.. There is just something so primal about sensing every movement, every breath, sensing this powerful energy of being so connected to reality and to your body.
It's exactly like sex.
Depending on the activity and the inherent risk of that activity, there may be more of an adrenaline component which can really give me a rush of aliveness.
If I need to feel my heart racing faster than ever before, knowing that I could crash and die at any moment, I'll speed down a mountain on a bicycle.
If I just want to relax and feel an ultimate sense of peace, I'll do yoga.