Saturday December 19 Workout

This is a pretty typical workout for Saturdays:

Bike MixType:
BikePlanned duration: 1:00
15 min PE2-end with 3 X 30 sec race pace
Main set: keep rpm 90+ throughout
1X 10 minutes at PE 4
2 minutes easy spin
1X 7.5 minutes at PE 6
2 minutes easy spin
1X 5 minutes at PE 7
Plz let me know corresponding HR's
Cool down PE 1-2 for the remainder of the 60 minute workout

Workout #2: T-runType:
RunPlanned duration: 0:20
Have your gear all laid out ahead of time..Spend no more than 3 minutes in Transition. Easy endurance run-
9:50 pace-i
intersperse 3 X 15 sec strides within...
Keep cadence at 90, and focus on perfect form, relaxed upper body

Now it does not seem that hard or time consuming but for some reason when I get to the run I am not able to keep the pace, I always go way too fast! I then realize I am going way too fast and try and slow down but by then the whole pace is off. I am not sure why I always do this. Does this happen to anyone else?