Since I do a lot of long distance (200k - 400k) I'm pretty familiar with all manners of avoiding, and treating saddle sores and chafing issues.
Since you're already wearing cycling shorts, first thing to determine is what's causing the sore.
Is it a chafe at the groin from the chamois pad rubbing at the side of the saddle?
Look into a pair of shorts with a smaller/thinner padding.
Is it at the contact point of your sit bone?
Saddle could be too high (as already mentioned.) It could just be that you need to use some chamois creme to reduce friction between you and the fabric.
Is it perineal?
Saddle angle is likely to blame, but an overly large chamois pad can contribute. Look for padding with a center channel, like the PI P.R.O. 3D/4D pad.
I find that for rides over 30 miles, especially if I'm going to be sweating a lot, chamois creme is a must. On the cheap, you can get away with using Udderly Smooth hand creme from the drugstore. It's inexpensive and reasonably thick so it isn't dissipated by a little bit of sweat. A step up in price is Chamois Butt'r, which IMO, isn't too much better for about triple the cost. An hour or two of sweating and it's pretty thinned out. Assos is the top-dog of chamois cremes, and it's a bit of getting used to since the combo of witch hazel and menthol is sort of like lighting your nards on fire and then putting them out in a bucket of liquid nitrogen. The consistency remains reasonably thick even after an hour or two of sweating.
If you're really having problems, ride for 10 or more hours at a time, or are currently nursing a saddle sore but can't keep from riding, then you might need to go for some of the more hardcore solutions.
Lantiseptic Skin Protectant. Put it on the affected area and use chamois creme on shorts as usual. Another option is
Amerigel.
I've been disappointed with just about every chamois creme except Assos, but I'm not going to spend the money for a case of the stuff, so I've formulated my own long-distance chamois goop.
1 cup Udder Butter hand creme
1/2 cup Petroleum Jelly
1 Tablespoon Aloe Vera gel
10 drops Witch Hazel
5 drops Menthol essential oil
Resistant to sweating, only needs minor reapplication about every 3 hours, anti-bacterial (menthol/witch hazel) and emmoliant (aloe vera).