21K Trainer is your Personal Trainer for Half Marathons. Completely flexible (and re-configurable) schedules based on your target time, current condition, number of trainings per week and time before your next race.

Features:
- Personalized Half Marathon training.
- Training program divided in 4-weeks cycles, each cycle includes 3 weeks of intense training followed by 1 week at reduced workload.
- Scheduled trainings can be fully edited (date, type of training, pace, number of intervals, etc.).
- Supports bluetooth smart (or low energy) heart rate monitors (e.g. Polar's H7, mio alpha and Armour39).
- Target feasibility estimation based on your current condition.
- Includes short intervals, long intervals, progressive, and long trainings.
- Monitor your performance with respect to the scheduled trainings and targets, get scores for each training.
- Track all your stats (pace, distance, time, heart rate, maps) and share them with your friends on Twitter and Facebook or via email.
- Calories counting.
- Advanced auto start/pause uses acceleration to reduce delays to 0.5 seconds.
- Vocal messages every mile or km, plus feedback on each training phase.

Please note that continued use of GPS running in the background can dramatically decrease battery life.