Best to post a picture of the seat with level camera to get a sense of your saddle tilt. As a general rule the 'rear' of the saddle should be close to level. This is not the same as placing a level over the entire seat. The nose should be in the air. It is also quite possible the saddle is no good for you. Most of us have gone through an exhaustive trial and error process. 5 miles is nothing I will add. I have been at this a long time but I can ride a century with little azz discomfort...rode 100 miles in the last couple of days...and butt feels fine. But...again...long time cyclist and countless equipment/fit tweaks to dial in my fit.
You will get there if you put in the effort.
PS: much can be written about butt pain on a bike. A lot of it is rider position. If your pelvis is upright, you put a lot of pressure on your sit bones. A racing seat is designed to ride with a tilted forward pelvis. A good recreational position on a flat bar or road bike is with a torso angle of close to 45 degrees. If you ride the bike like a cruiser, like I suspect based upon what you have written and the saddle is for a road bike, you are pretty bound to have sit bone pain...almost regardless of road bike saddle choice. Cruisers have a plush seat for a reason..and are wider designed for pelvis to sit like in a chair..and btw a road bike seat won't work well for this riding position.