Will Whey Protein give me any gains?

Since around November 2006 I have really been watching my weight and trying to slim down. I am 5'11 and went from around 180 to currently around 160. I work at a Fence Company, which is pretty hard work, so I have really been cutting back on my lifting and cardio because I'm beat at the end of the day. I probably lift light to medium 2 times during the week and play softball for cardio. I would like to begin building muscle back up and I'm wondering if this plan would have any real effect. I normally wake up on Saturday mornings and go for a 20-mile bike ride and then get a hard lift in. If I took Whey Protein after ONLY these Saturday workouts would it be basically worthless if I'm not ingesting the Protein all week long? I plan on Biking, Lifting Hard, and then finishing with a Whey Protein shake of some sort (milk, banana). Thank you for reading along with any information or comments you have.