Are the "butt sores" being mentioned in the question, a tenderized sit bones area?
A general guidance I give every new rider (and subscribe to myself if I have been away from riding for an extended time) is to limit a first ride to 30 minutes maximum. Then take one or even two days off to allow the tenderness in the inexperienced sit bones area to abate before riding again.
Continue this "sit bones conditioning phase," slowly increasing the duration of the rides and reducing the time between rides until the conditioning is complete and the rider's sit bones area is conditioned to supporting the rider without the accompanying pain/discomfort afterward.
This advice is especially helpful with riders new to the activity, as their early enthusiasm can be crushed before realizing the joy of making cycling a life-long passion. If they don't realize there is a conditioning period, they may blame the pain on themselves or the bike itself, when in fact, it just takes time to get past this phase.
One can rush this conditioning phase, say riding on consecutive days from the start. However, there is usually some pain. Initially it is worse, but it usually abates a little after 5-10 minutes. If one can tolerate this, the conditioning can be accelerated a bit. It is purely the rider's choice: Accelerate the conditioning of the sit bones area with some pain, or condition at a slower rate. Both methods will get to the end result: conditioned sit bones.
A properly fitted bike is important, but even when properly fitted, it takes a few rides to condition/break in the sit bones area for extended riding.