For me, it's all about strength. If you do a long endurance ride and feel weak, then you won't last long. If you have strength, then you will last longer and as a result your endurance increases. So it's simple, yes gradually increase your riding times on the bike, but work in some strength building exercises in the gym as part of your training program. You WILL see an improvement in both endurance and speed as a result. For me, before increasing my weight training, I was able to cruise at around 35-36km for a short time. After performing weight training (squats etc) for around 2 months, I was able to maintain that speed for longer, and my sprinting speed increased to 55kph and even up to 70kph on the flat with a strong wind on my back!! My trainer in the gym I go to told me the benefits of weights but I didn't believe him that it would make so much of a difference but I'm now converted. I'm training for the Etape next year and weight training is at the moment priority number one for me and cycling 2nd.