I am currently at the beginning of a 12 week half marathon training course. I haven't 100% committed myself to the HM yet because I want to see what shape I am in after 4 weeks, but it is looking good so far.
On Saturday I ran 5km (fast pace), Sunday I ran 8km (slow pace) and today almost 7km (intervals).
Today I ran 1 mile at a slow pace to warm up and then did 6 sets of 400m sprints and 200m recovery walks. I then finished with a 1 mile slow pace cool down.
I am using a heart rate monitor and the programme I am following was saying I needed to get up to 95% of my maximum heart rate during the intervals, it was really tough to keep that pace up for 400m but I think that is the point of sprints. It supposedly raises the threshold at which your body produces lactic acid, this allows you to run at a faster pace for longer.
I have posted a link of my run below and it is very obvious when the intervals are. It says my best pace was 3 min per km, but I can assure you I only kept that pace up for about 10 seconds! My average during the sprints was about 4:25 mins per km.
The programme has one interval session per week so hopefully I can improve over time and this will feed into my normal runs.