Running Tip #1:
When training for a race, or for general fitness, training in three week cycles can be extremely beneficial.
I-I-P: Introduce. Improve. Perfect.
During the first week, introduce a new speed workout (such as a tempo-run, intervals, or hill repeats) Without over exerting yourself, start off small. Slightly slower than goal pace, slightly shorter in duration. The goal of this run is to get your legs and body accustomed to the new added stress of the speed to come.
During the next week, improve from your previous workout. Say you went out for a tempo-run with 3 miles at 7:00 pace during your introduction phase, this week build upon the prior week by going 4 miles at 6:50 pace.
During the third and final week of the training cycle, perfect that given workout. Continuing on with the example of a tempo-run training cycle, perfect the workout by going 5 miles at a goal pace of 6:45.
After the three week cycle, you can continue on to another cycle of training, such as interval training, fartleks, or hill training. Mixed in with long runs, recovery runs, and easy paced runs, you will be well on your way to setting new PRs.
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