I consider myself to be a newbie to speedwork. I've tried a few different types of workouts:
Fartlek ("speed play"): Random bursts of speed of varying time or distance scattered throughout the run.
Tempo runs (Hal Higdon style): 10-15 min easy running, followed by a continuous buildup to half marathon race pace. Peak speed comes about 2/3 into the run, and is only held for a few minutes. Cool down of 5-10 min easy running.
Tempo runs (as others describe it): 10-15 min easy, up to 40 min of 1/2 marathon race pace, 5-10 min cool down.
Intervals or repeats (I get the two mixed up, but I understand them to be the same): 10-15 min easy followed by 400m or 800m of a certain speed, with about 1.5-2.5 min rests (jog or walk) in between. For 400m intervals, I would run at 5k race pace; for 800m intervals, I would run 10k race pace. Usually I start with 4 intervals at the beginning of my training cycle and add on each week until I get to 10.
I have done 90% of my speed workouts on the treadmill. It is just much easier to keep myself on pace, and I don't have to deal with any terrain changes. I've gone to a track for interval workouts once or twice. If I try to incorporate speedwork into an outdoor run, I find that I have trouble pacing myself.
Take today, for example. I headed out with the intention of doing a fartlek. After about 10 min of easy jogging, I ran some 2-3 min intervals at a faster pace, with 1.5-2 min rest(jog or walk) in between. This is what happened (time, pace):
interval 1 - 2:06, 5:50/km
interval 2 - 2:39, 5:35/km
interval 3 - 2:27, 5:15/km
interval 4 - 2:28, 5:01/km
By the end of the last interval, I was gasping and it kind of screwed up the rest of my run. I could not have done another interval. Obviously, I have some trouble with pacing. It's like I was trying to compete with myself by making each interval faster, rather than keep a steady pace. I think that I should have tried to stay in the 5:15-5:30/km range. My Garmin does not really help me stay on pace, as it seems to lag a bit when I speed up, and I always see that I am running slower than reality (probably another reason why I kept running faster). Perhaps I should pay more attention to my perceived exertion!
Next week, I will attempt to do my tempo run outdoors. We will see what happens with my pace!