See this fartlek running page for guidance on how to run fartlek.
Your long run will be 12-14 miles each week. You should try to avoid running more than one third of your mileage in one run. Alternate the 14 mile run with the 12 mile run for variety. Both runs will improve your circulatory system and oxygen uptake as explained at:
10k & 5K Running, training & racing recommends that all runners do speedwork, especially the beginners. New runners need to learn good running form...by running fairly fast while building mileage. Do not sprint. Do run fast enough to get somewhat out of breath, run easy, then run fairly fast again.
Serious 5K runners (or beginner runners in other authors' books), will run fast once a week, starting with about 2 miles of fast running, building up to 4 miles by the end of 10 weeks.
Beginner runners or those new to fast running will keep most efforts short to develop running form, then gradually introduce some longer efforts into the fartlek running. After running about 10 sessions, you should be running two miles of the long efforts (up to half a mile each) and 2 miles of the short efforts. Speed should be no faster than 5K race pace.
Day one: easy 8 mile run Day two : 12-14 mile easy pace run Day three: easy 4 mile run Day four: rest Day five: Four miles of fartlek running, plus warm-up and cooldown Day six: easy 8 mile run Day seven: rest Total mileage is 40-42
The above running schedule should be repeated for 8-10 weeks using a variety of surfaces for your fartlek running. Incorporate small hills, or running in dirt or sand for additional strength.
New runners and joggers will run their single fartlek session three days after the longest run, thus gaining the most benefit from the hard running, easy running extolled by most coaches.
Before moving on to the remaining four phases of 5K training, enjoy a race or two. Chop two miles off of all 5 runs; run three miles of fartlek instead of four: make sure you don't run any faster in training due to having fresher legs, then use those fresh legs to run a decent 5K race.
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This is the fartlek schedule page based on 10K & 5K Running, Training & Racing, by David Holt.