In my opinion if you are good for 40 miles it’s not the saddle but more of conditioning yourself for more miles in the saddle at one time that only comes from putting in the mileage and learning to move around some and stand at regular intervals to relieve the area. This all needs to be done before you start getting sore so start standing well before you start to fill anything it doesn’t have to be long just 30 sec. at a time can make the world of difference.
If the saddle is really wrong it happens way before 40 miles.