Enforced Fartlek training

What is Fartlek training?
Here's a quote from Wikipedia and the link if you'd like to know more: http://en.wikipedia.org/wiki/Fartlek  


Fartlek, which means "speed play" in Swedish,^[1] is a form of interval training which puts stress on the whole aerobic energy system due to the continuous nature of the exercise. The difference between this type of training and continuous training is that the intensity or speed of the exercise varies, meaning that aerobic and anaerobic systems can be put under stress. It differs from traditional interval training in that it is unstructured; intensity and/or speed can be varied whenever the athlete wishes......


Do I like it?
Yes I do and I ought to do more!  This is when I am running and when I'm warmed up, so probably not before the 20 minute stage.  This is what happens: I'll spot a point ahead of me and force myself to sprint - as fast as I can - to that point.  The point could be anything - a tree, a parked blue car, a lamp post, a...whatever but normally about 100 metres away.  


So, what do I mean about "enforced" fartlek training?
A few days ago I was having a fairly gentle run and coming back over a nearby hill.  Imagine those deep ridges you sometimes find along the side of a hill, like a little valley.   I had just started to run down one of these, with a fence to my left and straight ahead was a steep climb - perhaps 1 in 4 which would last for about 1 minute.  Then I spotted some nearby cattle and, more to the point, they had spotted me.  Most weren't at all bothered as I was not heading directly towards them.  There was one, a bull that stood up, alert looking and staring intently at me.


I sprinted as fast as I could going down and the last few steps before the climb had me looking over towards the bull, hoping he wouldn't react.  It was then I realised I had my bright red running shirt on and I tried to tell myself "red rags and bulls are a myth".  Adrenalin kicked in and I made it to the top so fast I amazed myself.  I looked back to see if the bull was behind me.  I was almost fearful to do so but I knew he couldn't make it up as quickly as me, surely this is right, isn't it?  There was no sign of him.  Phew.  Perhaps a lucky escape.


My heart was beating fast but not pounding but it quickly eased back while I carried on at a jogging gentle pace while I got my breath back and it felt good. My heart rate completely settled back down and I picked up my regular pace once again and felt chuffed I had survived and had the story to tell.


I didn't plan on this extra sprint but I'm so pleased I did it.  It felt good afterwards, those endorphins causing a rush of a wonderful "runner's high" sensation.  These intense periods of activity, raising the heart rate for just a short period, are so important in training for races and endurance events.  I must do more while I run and I'm sure I will reap the benefits with cycling.  Improving performance now - in realistic amounts - will be of benefit in years to come.


Resources
There are many reasons to vary the pace at which I exercise, no matter whether it's with running or cycling.  Here's a few worth following up:
http://www.momentumsports.co.uk/TtFartlek.asp
http://www.notyouraveragefitnesstips.com/best-workout-routines/advantages-of-fartlek-training-session-interval-training-workout-improve-anaerobic-aerobic-endurance