Race time predictions are most accurate for adjacent distances (5K to 10K, 10K to half marathon). Predictions become less reliable when extrapolating across very different distances or when you haven't trained specifically for the target distance. Training specificity, race experience, and individual physiology all affect accuracy.

VDOT is a measure of running fitness developed by exercise physiologist Jack Daniels. It combines VO2max (aerobic capacity) and running economy into a single number. A higher VDOT indicates better running fitness. It's calculated from your race performance using formulas that account for the oxygen cost of running at various speeds.

Predictions assume equal training for all distances. A pure 5K runner may not hit predicted marathon times without long-run training. Similarly, a marathoner's predicted 1500m time may be unrealistic without speedwork. Course difficulty, weather, race-day execution, and pacing strategy also affect results.

Yes, but more recent races and distances closer to your goal provide better predictions. A 5K from last week predicts your current 10K fitness better than a marathon from two years ago. For best results, use a recent all-out effort at a distance you train for regularly.

VDOT improves with consistent training that develops both aerobic capacity and running economy. Key workouts include: tempo runs (threshold pace), interval training (VO2max development), easy runs (aerobic base), and long runs (endurance). Most runners can improve 1-3 VDOT points per year with dedicated training.