Strength training for runners is a passport to better running. Better running means running without injuries and seeing progress with speed over time. Strength has a key impact on your health, fitness and running performance. Improvement in running performance needs runner-specific strength training. A strong core is key ingredient of a distance runner. Here are some exercise that you can do once or twice a week depending on your capacity. Do these during your easy running days or off days.
The above is a partial list and a good list to start with.
Suggested duration for Plank and Squats against the wall are 20 to 30 secs and repeat 3 to 4 times. Whereas do pushups and lunges 8-12 reps, 3 sets each. These are best done as circuit training with a minute rest between the different exercise.