Aim for two or three resistance training sessions a week, resting at least a day between bouts. This will help you maintain strength and reduce your risk for injury as you increase mileage.
Since you run with your whole body, and not just your legs, strengthen it all in the weight room with basic movements in which you push (e.g. bench press, squat) and pull (e.g. row, Romanian deadlift) with your upper body and lower body as well as rotate (e.g. chop).
Also, be sure to include exercises that will improve your posture and core stabilization to help you stay tall as you run. Try performing pillar bridges and either cable lifts or cable chops in each training session. So if you're training on Monday, Wednesday, and Friday, for instance, you might do chops on Monday and Friday and lifts on Wednesday, and then alternate the following week.
-- Paul Robbins, Metabolic Specialist, Athletes' Performance