I don't know what bike you get or what saddles are you using but since it is your first ride, it might not be the saddle. Even if you buy an expensive gel saddle, it will not help. Most probably, I think is your butt has not got used to it. And also you were pushing the limits a bit. Pushing the ,limits is not your stamina but more on how long can your butt withstand sitting on the saddle. OFF COURSE a good saddle helps though. Some can pedal at high resistance for 3 hours in a gym but once on the road, it is a different matter. Just ride slowly and enjoy first. Then increase time gradually and see if it still pains.

One thing I know is, after so long of riding, my butt still pains if I push too hard. Once it pains, you have to endure it the whole ride!