Keep The Hard Days Hard
The philosophy behind keeping your hard days hard and your easy days easy is simple. You want to do your hardest strength training workouts on your hardest running workout days so that your easy days remain as easy as possible and recovery can take place.
On the other hand, performing harder strength-training workouts, especially anything that involves the lower body, on your easy running days means you take away from your body’s ability to recover by adding stress and by shortening the total time you have to recover before your next hard workout. This is the single most important reason to include strength training on your hard workout days.
In addition to allowing you to properly recover between workouts, performing your hard strength training days on your workout days also provides a few additional benefits:
It prevents you from going too hard during strength training
Since running is the most important part of the training plan, it should be the primary focus and consume the most of your available energy and attention. While this does leave you more tired for your strength workout — and consequently won’t allow you to be as strong or explosive as you would like — it’s actually a positive. Being tired going into a strength workout will prevent you from going too hard or lifting too heavy, which happens too often when runners are fresh and hit the weights.
It burns more calories and aids in recovery
Scientists from Brigham Young University found that post-exercise metabolism increased most when people did intense cardio first and lifted weights afterward. This means that you’ll burn more calories, and burn them for longer, if you do your strength training after your more intense running sessions.
Likewise, researchers from the College of New Jersey found that following weight training, heart rate and blood lactic acid returned to resting levels faster, which means you could potentially recover from hard running faster if you perform strength training that day.