I have a two year old male Shepherd who INSISTS on his daily walks - so I use his morning walk as a warm up for my bike! When we get home, he gets his breakfast, I hop on my bike and he takes his nap. Everybody is happy!

Be sure you don't push really tough gears on your bike for awhile. Your muscles will get stronger much faster than your tendons & ligaments. Spinning at ~ 90 RPM will exercise your heart, raise your metabolism and keep your knees happy. Intervals are your friend for losing weight. Be sure that you incorporate some weight training and watch your diet. Don't use your cycling workout as an excuse to load up on carbs. As you get fitter, your body will learn to burn fat preferentially over sugar.

And be careful about whining about being "old" - 47 around these parts is just about right!