So my galaxy watch 4 classic usually goes from 100% first thing in the morning (6am) to about 40% battery left by 10pm after a day which includes 1.5 2.5 hours of exercise.
That sounds terrible. Did you update any settings like stress and heart rate monitoring ( set them to measure continuously) ? Is the screen brightness high and is the screen set to be always on? Wifi/GPS/NFC - don't keep them turned on. These are some of the biggest culprits when it comes to battery consumption.