I recommend the "Tips for Riding a Century" thread. It has a lot of relevant information.
One of the important habits to learn is to eat and drink regularly throughout the ride (at least 200 kcal an hour, at least some water every 15 minutes).
Personally, I started road biking in October after riding as much as 50 km on a shopping bike. I did rides of about 100 km in December, about 160 km in February and my first 300 km in May. Riding further is definitely easier than riding faster.