This task can be performed using Watchletic
The Apple Watch app for serious runners
Best product for this task

Watchletic
lifestyle
The app for when you want to take your running to the next level without having to purchase a Garmin watch. It integrates with many external services (TrainingPeaks, TrainAsONE, etc.), lets you setup and run structured workouts, advanced features like pacing target splits with reference routes for hills, scheduling workouts, outside target range alerts, etc. Perfect for planning your next marathon race, and the training to get there. All in a simple to understand UI, no more complex setup!

What to expect from an ideal product
- Here's a bullet list of 5 items about how Watchletic solves the question of setting up customized pace targets for marathon training:
- Allows creation of structured workouts with specific pace goals
- Integrates hill information for more accurate pace targets on varied terrain
- Provides outside target range alerts to keep you on track during runs
- Offers scheduling features to plan your entire marathon training program
- Syncs with popular training platforms for a seamless experience