Strength training should be high rep low weight to imp[rove running performance
The facts in this are that strength training completed 2-3 times a week for 6 weeks or more improves endurance running performance, running economy and maximal sprint speed
There is a belief that to maximise performance benefits, strength training should be high rep and low resistance to mimic the endurance demands of longer distance running.
This is INCORRECT. Greater performance benefits are reported with heavy and explosive strength training compared to strength endurance training.
The key to all of this is consistency. Performance benefits are likely to be greater the longer a program is undertaken. More significantly they are lost quickly once such training stops.
So what should you do?
Most importantly seek the advice of an experienced fitness professional or coach who has worked with runners. The purpose is two-fold; first to ensure your training is controlled and organised and second to make sure that, if you have never trained under heavy loads before, that you perform the various exercises correctly.
Examples of the types of training to undergo are: barbell squats, deadlifts, step-ups, lunges and calf raises.
The loads need to be anywhere between 60% and 80% of your 1 rep max (heaviest weight you can lift for 1 rep), hence the need for a trainer. These should be 3-6 sets of 5-15 reps to get the optimum performance benefits.
Finally and just as important, the programming should include adequate recovery time between running and resistance training. This is more than 3 hours.
(With acknowledgement to BJSM MYTH and TREK)