If you are used to a specific distance, then going a bit further, you just have to keep it under 'the boil' - e.g. My rides are upto 40 miles (60k) and maybe 3 hours. More often rides are 90 minutes as fast as I can, or the 20 miles each day commuting.
Recently spent two consecutive days riding for about 8 hours each day off road. The key was to keep the climbing speed (off road) under the point you were starting to push really hard, as too much pushing, like you'd do on a short ride, would have you spent. You need to ride at a pace that feels like you can 'keep it going all day'. You also have to take into account any injuries and what the extra time in the saddle will result in - e.g. my back isn't brilliant after a broken spine 8 years ago, so I have to make sure I stretch every so often.
Drink plenty, 500ml an hour as a guide, and eat a little something. I really struggle to eat on long rides, my stomach doesn't like it, so I tend to use carb drinks instead. Do what works for you.