Getting started with long distance rides...

Carry sufficient nutrition. I only really discovered how much I need to eat/drink while riding by trial and error. Bonking/hitting the wall is horrible, especially when you're miles from home.

Padded undercrackers.
Both good advice.
As for distance, as others have said; take a distance you know you are comfortable with and add a bit to it. Personally, I find that increasing by 10 miles at a time is a good step, as it only adds about another 30 mins to a ride. Then do one or two "normal" rides, then add another 10 miles the following week. Each time you do it, increase your "normal" distance.

Everyones different so if something doesn't work, try something else. I have read that it is recommended to take your base weekly mileage and increase it by no more than 10% per week, not tried anything this disciplined myself though.