A century may seem like a lot but if you can bike at 15 mph for 6.7 hours then you will travel 100 miles. Best advice is to warm up for 20-30 minutes and drink water to stay hydrated and eat snacks that provide energy (raisins work very well).
On hills as it is not a race take a pace that is comfortable and when choosing between two gears use the lower one. Easy to psych oneself if thinking that you have arrived at the top of a grade and then round a turn and find it continues further. Using a lower gear to conserve yourself makes this less daunting and bicycling is as much mental as it is physical.
For clothing I like the bicycling jackets that have a wind shield on the front and lots of side and back vents and that have removable sleeves. I wear it over a synthetic material bike jersey and depending on the weather it may be a short or a long sleeve jersey.
It is more about duration than speed or distance as such.