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The training science behind Trainer. These are the same notes Claude works from when it writes your plan, so they explain why your plan looks the way it does.

• Training zones and sessions

  Trainer uses Jack Daniels' VDOT system. VDOT is a fitness score worked out from a recent race or hard run (see Fitness and VDOT), and each training zone is a pace derived from it. Plans name a zone ("at T pace") rather than a number, and the app shows your pace for that zone beside each session. As your fitness changes, the paces change with it.

• Intensity distribution: mostly easy

  Successful endurance athletes do most of their training at low intensity and only a small share hard. This pattern is often summarised as "80/20": about 80% easy and 20% moderate-to-hard.

• Progression, load and recovery

  Running injuries mostly come from doing too much too soon. The safest plans build gradually from what you're actually doing now, not from what you'd like to be doing.

• Periodisation and tapering

  Good plans change emphasis over time, from general fitness towards the specific demands of your goal, and then ease off so you arrive at race day fresh. This is called periodisation.

• Fitness and VDOT

  Trainer measures your running fitness with VDOT, and uses it to set your training paces, predict race times and judge how realistic a goal is.

• Heart rate and recovery

  Heart rate tells you how hard you're working, and your body's numbers between runs give clues about how well you're recovering. Trainer uses heart rate to check that easy runs are easy, and shows recovery trends so you can spot when to back off.