Hi Jon.
I recommend sticking to the plan and keeping the longest rides at 6 hours. This keeps you in the fitness building mode. Rides under 6 hours build fitness and rides over 6 hours spend fitness.
What you are asking about is the “gap”. This is the difference between your longest training ride and your race finish time. In endurance MTB racing there will always be a “gap” – that is ok. Keep the focus and priority on fitness building. A 6-hour ride is enough time to prepare you to go all day and allows you to execute other training goals in a training week.
Coach Lynda