If you are training for your a half marathon or 21K you can benefit from following a half marathon training program for developed by an Australian endurance coach. Ben Wesby
The 12 week program contains training tips and ideas for half marathon training. It covers the following areas :-
1. Long runs for half marathon
2. Interval training for half marathon
3. Strength sessions for half marathon
4. Speed work for half marathon
5. Improving your lung capacity
You can download the detailed PDF report at half marathon training program site.