Fartlek literally means 'speed play' in Swedish, and it really is just that...playing with speed. Fartlek sessions are a work-out both aerobic and anaerobic fitness levels and as a result are great for increasing endurance and overall running performance.
Fartlek sessions are different to interval sprints in that they are continuous. Intervals require stopping and starting whereas is fartlek sessions you vary your pace between jogging and running faster segments, but you will always be moving. In interval sprints you have a set distance that you stick to and set recovery period. However in this type of training the slow-fast intervals are determined by how the athlete feels (yes you are an athlete).
To incorporate fartlek training into your runs, every so often throughout a longer run you should pick up the pace to faster than you would run on race day, and maintain this for a minute or so, then drop back down to slower than your normal running pace until you catch your breath. Once ready start running at normal pace again, then after a while repeat the process. Again this is not based on a set distance, so just respond to how your body is feeling. You will have to push it in the faster intervals, but if you need longer to recover that's completely fine.
Fartleks will put extra strain on your system and will raise both aerobic fitness and anaerobic threshold as mentioned above. Plus its a fun word to say. This is also a great way to lose weight, so if that is a concern, smack a few fartlek training sessions into your schedule and watch the weight drop off.
