Some wisdom from coach and former world record holder Alberto Salazar
http://www.outsideonline.com/fitness/running/Alberto-Salazars-Ten-Golden-Rules.html
http://www.nyrr.org/about-us/nyrr-hall-of-fame/alberto-salazar
In summary:
- Train consistently, 4 days a week is good
- Recovery is an important part of training
- Increase distance gradually, no more than 10% per month
- Run on soft surfaces (grass, dirt, chips) as much as possible
- Train faster than you want to run in a race
- Work out your whole body, including upper body and core
- Get the right shoes
- Pay attention to form and inefficiencies
- Test your limits once in a while
- Collect data so you can compare runs over time