Question: What is your goal? It is to ride faster generally, or is it to finish 100km or 100miles in good order with out the shame of sitting in the sag wagon?
I think to do long rides you have to ride long rides regularly in your training. I like to work up to 2/3~3/4 of the distance of the event/distance I want to ride at slightly higher pace I would do at the event. That is if I want to ride the local century I'll try to work up to at least 70miles during training at maybe 5% more effort then I want to ride for the event. Do this slowly starting at maybe a 2 hour ride and then ladder up week by week to your goal. To prevent fatigue from creeping in you can do 'the long ride' every other week.
During the actual event pacing is so very important. Don't get excited and ride like crazy from the start. My rule is to treat the first hour as a 'warm up' and ride pretty easy before going harder or joining a paceline.
Long rides over a 3 month period look like this while working up to the event:
week, miles
1 30m (~2 hour elapsed time)
2 35m
3 40m
4 45m (or what 3 hours total elapsed time gets you).
5 50m
6 60m
7 45m (the 3 hour ride)
8 65m
9 45m
10 70m
11 45m
12 - Event/ Goal ride