Here’s some advice from Jeff Galloway of Runner’s World

At some point, almost every runner wants to run farther. You complete one mile, you want to try two. You reach three miles, five seems possible. Adding distance to your routine is smart because it improves fitness, builds endurance, and burns calories. Every other week, try running “long”–that is, any distance beyond your usual miles. If you usually run three, start at four miles. Your goal might be to reach an hour, run 10 miles, or work toward a marathon. The following guidelines will help you build miles without undue fatigue or injury.

Slow down
Taking your speed down a notch or two gives you energy to go longer. Add about two minutes to your usual pace. For example, if you usually run three miles in 30 minutes (10 minutes per mile), expect to run four miles in 48 minutes (12 minutes per mile) or longer.

Build gradually
Adding miles too quickly can lead to burnout and injury. So increase your long run by no more than one to one and half miles at a time.

Run long every other week
This gives your body time to recover, which reduces injury risk.

Take walk breaks
A six-mile run with walk breaks still gives you six miles of endurance. The benefit? More energy for your run and a faster recovery.

Run-Walk Guide:
The chart below shows a suggested breakdown of running to walking based on your pace.

Typical pace per mile 8:00
Run-walk breakdown 6 min. / 1 min.

Typical pace per mile 9:00
Run-walk breakdown 5 min. / 1 min.

Typical pace per mile 10:00
Run-walk breakdown 4 min. / 1 min.

Typical pace per mile 11:00
Run-walk breakdown 3 min. / 1 min.

Typical pace per mile 12:00
Run-walk breakdown 2 min. / 1 min.

Typical pace per mile 13:00
Run-walk breakdown 1 min. / 1 min.

Typical pace per mile 14:00
Run-walk breakdown 30 sec. / 30 sec.

Typical pace per mile 15:00
Run-walk breakdown 30 sec. / 45 sec.