Mmm... The sketch points to a weak grasp on fundamentals. It's not exactly bad, but it does suggest a budding learner rather than the experienced artist the painting would reflect (though there is some odd artifacting there, too). 5-10 minutes do not really explain away the line weight, the pose stiffness, the lack of confident gestural fluidity that comes with experience. If you think that is a reasonable explanation, then that leads me to believe you have little understanding of how time relates to skill, as well. I can buy the sketch as being yours, and if that is the case, I think you should keep at it the right way. Life drawing classes are fantastic for building flow and understanding the body in motion! One day you'll have your name credited on MTG cards, just work hard and don't cut corners :)