Running a half marathon, marathon or even a 10k becomes exponentially harder when you add the difficulty of uneven terrain and elevation gains. There is no exact math here as the fun of mountain trail running is the uniqueness of every trail. But, keep in mind that, at times, your pace may be double your normal road pace.
If you are a normal 4 hour finisher for a road marathon, running at a 9-minute mile, your per mile time could go up to a 13-minute mile which would have you finishing in 5 hours and 41 minutes when running a trail marathon.
When running trail races—especially a long distance of a marathon—pace becomes less important and you may even want to stop tracking pace for a bit if you tend to be really pace focused.
Whether this is your first marathon or you are an advanced runner, the game changes when you get out on the trail. You will be demanding more of your heart, lungs, leg muscles and core throughout the race and it is important to train accordingly.
Make sure to get off the road and onto trails during your training sessions to locations with elevation changes and different terrains. In addition to running, your training schedule will need to include strength training to increase your overall fitness level.
Bodyweight training is a great place to start with things like squats, lunges, step-ups, even pushups and pulls-ups. As you get stronger, you can begin to utilize resistance bands and kettlebells.