When I compete in the 5k races I have learned not to start out too fast.  I used to get caught up in  runners going by me and I would try to keep up only to lose a certain amount of steam later in the race.  The more races I have run I realize other joggers must feel the same because I see them dropping like flys within the second mile.    Of course I throw this theory out the window when I see the time clock up ahead and go as fast as my body will carry me at the end of each race!   I run my personal best when I control my breathing.  Also I keep my stride the same even when faced with steep inclines/declines in the road.    This is just something I have learned about myself over the years.  When I start out on a daily run or a weekend race, I maintain a comfortable pace.  I get my breathing in check within the first .1 mile mark.  This sets the pace for the entire run and I feel great and actually enjoy it.  I have had times where I completely get ahead of myself and realize I am sprinting.  This simply is not condusive with running a few miles.  Breathing gets erradict and I need to stop or slow down.  When the weather is cool and crisp I feel the desire to run fast.  I just love certain times of the year and feel as though I can go the distance at an above average speed (speed for MY body)  But over the years I have learned that by pacing myself I am able to keep enjoying and looking forward to my daily jaunts.  Aside from stretching, and a great pair of running shoes, I can truly love my time running when I regulate my breathing.