If a long ride is uncomfy and you don't habitually do long rides, the discomfort is probably due to the unfamiliar activity. If a long ride is uncomfy and you're developing problems like chafing, saddle sores or other medical issues, then it's fair to blame the saddle (or seam rub, or elastic rub...).

It sounds like your current saddle is Good Enough, so getting used to long rides on it is a good goal . Might be good to take your breaks *before* your rear is tired and needs it desperately. I find with repetitive activities, my body is a lot happier if I take breaks before it's urgent.