Be REALLY careful about anything that's going to fool with your metabolism. And "diets" do - which is why dieters lose & then gain what they lost plus interest. Is there anything about your eating habits that you can just clean up a little, or shift in terms of eating smaller amounts, more often? More veggies, an earlier dinner?

Also, it won't take a 20 lb loss for you to feel lighter and leaner as you climb the hills. That is a great deal of weight, and you don't want to lose your strength or health (or peace of mind) with it! I bet even 4 or 5 lbs loss would feel quite different. As my favorite trainer says, "Eeeasy, princess!"