Congrats on making the decision to ride, and on the nice bike choice!
You have put on a good amount of miles in the last two weeks, and though 15 miles doesn't sound like a lot for experienced riders, the 25 miles you did in two days can be quite a lot for a new rider. Especially their behinds. The soft tissue can become bruised, and it takes some time for the pain to ease. I would lay off riding for a bit until the pain and tenderness subside somewhat. Many people get saddle sore on their first rides of the season, even on well-used saddles that are normally fine for them. It can take a bit of riding to build up some calluses, or desensitivity in that area.
You also have a new saddle, which takes time to break in. Were you fitted to the bike with this new saddle, having the position adjusted fore and aft, the tilt, and height? It can be something as little as the tilt being off,and your sit bones not hitting the right spot, to make you sore. The position and size of the saddle need to be just right. They come in widths to match your sit bones, so if your sit bones aren't hitting the right spot on the saddle you're going to get sore. Or it could be the saddle itself, if it's too soft (as in too much gel) it will make you more sore on longer rides because the soft tissue gets more pressure. Everyone's backside is different, so what one person swears by in a saddle another person doesn't like. There are numerous threads on here about "what's the best saddle", but take them with a grain of salt. For example, you wouldn't want a road saddle on your 8.5, as your body position is different and therefore so is your backside's position. The road bike saddle would create pressure points in the wrong spot. So it's important to have the right kind of saddle for the type of bike you ride.
And those bike shorts you bought should also help, provided you got padded ones with a good chamois. At this point I would give my backside a chance to heal, then ride with your new shorts, perhaps slowly increasing your mileage as comfort allows.