Let me guess: (1) Western saddle - a Western saddle that doesn't fit properly is much more uncomfortable than an English ditto (2) you SAT in the saddle instead of "standing" in it (if you're riding correctly your butt is in the seat, but your weight is tipped forward onto your inner thighs. Shorten stirrups as appropriate to maintain position.)
I hadn't ridden since before Christmas when I went out on a fox hunt for three hours last weekend - galloping and jumping through fields, woods, and soggy river bottoms. My quads and adductor muscles (on the front and inner thigh) were VERY sore, but my bottom was unscathed.
That's why you need to go to a trainer! When my non-riding friends announce that they are going pack-trailing or to a dude ranch, I set them up for a 6 week course of riding lessons before they leave. That way they can enjoy the scenery instead of shifting around in agony on what feels like a red-hot steel plate . . .
Anyway, don't judge riding by a jaunt on a Hertz Rent-A-Horse. Those horses usually aren't much good, they pick up lots of bad habits and most of their riders don't know how to correct them, it's just about the worst place to ride for a beginner.