What marathon athletes need to track
Marathon training is a multi-month commitment that hinges on one question: are you accumulating fitness or accumulating fatigue? Most runners train by feel - they run the prescribed distance in their plan and trust the process. But without objective data, it's impossible to distinguish productive training stress from accumulating fatigue that will force you to miss race day.
Baseline gives marathoners a unified view of their training load across every source - Strava, Garmin, Apple Watch, WHOOP, or Coros. Instead of piecing together your running log from three apps, you see your CTL (fitness) and ATL (fatigue) curves in one place, correlated with your sleep and recovery. The key metrics for marathon training are your long-run pace decay (how much you slow after 90 minutes), your heart rate drift (aerobic decoupling) at easy pace, and your critical pace at the marathon distance. These numbers tell you whether your long runs are building durability or just building soreness. Baseline surfaces the trend, not just today's number, so you can make informed decisions about your training week - without needing a coach on retainer.
Goal-specific KPIs Baseline surfaces
Training Load (CTL/ATL/TSB). Chronic Training Load (CTL) measures your rolling 42-day average training stress; Acute Training Load (ATL) tracks the last 7 days. The difference is Training Stress Balance (TSB), which tells you whether you're fresh, fatigued, or overtrained. Marathoners live in negative TSB during peak weeks and taper toward positive TSB on race day. Baseline plots all three curves and shows how your marathon pace workouts fit into your overall load.
Aerobic decoupling. As you run, your heart rate drifts upward even when pace stays constant. The rate of drift - decoupling - measures aerobic fitness. Less drift means better endurance. For marathoners, decoupling during a 90-minute easy run should be under 5%. Baseline tracks decoupling trends over time so you can see whether your weekend long runs are building durability or just piling on stress.
Critical pace at marathon distance. Your critical pace is the maximum speed you can sustain for a given duration. Baseline calculates your critical pace curve from your recent best efforts at 5K, 10K, half marathon, and longer. For marathoners, the shape of the curve reveals your strengths and weaknesses - a steep drop at marathon distance means your endurance needs work, while a flat curve means your marathon pace is close to your half-marathon pace.
Long-run pace decay. The most specific marathon metric is how much your pace degrades past 90 minutes. Baseline splits every long run into 15-minute segments and plots your pace for each segment. If you're fading more than 8-10% from your goal marathon pace by the end, you need more long-run volume or better fueling. The trend shows whether your endurance is improving run by run.
Recovery readiness (HRV, resting HR, sleep). A marathon block isn't just about the miles you run - it's about how well you absorb them. Baseline surfaces your overnight HRV and resting heart rate trends from WHOOP, Oura, or Garmin alongside your training load. When your HRV drops and resting HR rises while CTL is climbing, you're flirting with overtraining. When the trend reverses during taper, you know your body is ready.
Weekly volume and intensity distribution. Marathon plans prescribe easy runs, threshold workouts, and long runs. Baseline shows your weekly mileage alongside the Polarised (80/20) distribution - how much time you spent in Zone 1-2 vs. Zone 4+. Most runners are surprised by how much time they actually spend in Zone 3, the grey zone that feels productive but doesn't drive adaptation.
Correlation between sleep and run quality. Baseline connects your sleep data from WHOOP, Oura, or Apple Health with your running performance. Does a 7-hour night consistently produce better interval splits than a 5-hour night? The data builds a personal pattern over weeks. For marathoners in peak weeks, this correlation can be the difference between hitting target splits and blowing up.
Race time predictor. Based on your recent best efforts and your critical pace curve, Baseline estimates your marathon finish time using the Riegel formula. Unlike fixed calculators, the predictor accounts for your training load context and your personal pace-decay curve. Available as a standalone tool at /tools/race-time-predictor.
Recommended pricing tier for this goal
Marathon training generates data from multiple devices - your GPS watch for runs, a WHOOP or Oura ring for recovery, and potentially a bike for cross-training. The best plan for marathoners is Baseline Pro at $12/month or $249 lifetime, which unlocks unlimited device integrations, AI-powered daily insights, the full-resolution heatmap to review long-run routes, geographic stats to compare training camps, and the achievement system with 50+ badges. A marathon block is 12-20 weeks of concentrated training; the cost of Pro is less than a single pair of race-day shoes.