Take a look around the forums for information relating to bike fit, nutrition, and cadence/changing gears. These wont get you into better shape, but they'll make whatever fitness you have go farther. Google: "bike fit", "bonking cycling"/"randonneuring nutrition", and "cadence cycling".

The trick to endurance is expending as little effort as possible, but, unless you expend energy, you wont build endurance. It's an ironic activity. In the end it's a combination of technique, equipment, fitness, habits, posture, and states of mind that come together to produce a sustainable performance. You need to spend lots of time in the saddle trying to do as little as possible while simultaneously increasing speed and distance each time out. Do the aerobic thing and the weight training thing in between rides, but when you get out on the bike, think lazy, steady and comfortable, and keep thinking it for as long and as far as possible. Your model isn't an athlete performing an event, but a hunter cautiously tracking a wounded animal across the Savannah. Conserve energy. Be practical. Be calm. Be steady.