Setting up intervals for speed work on Garmin 305 Forerunner

Hit the mode button.

Then, Training>Workouts>Interval

You can select to do your interval reps based on:
Distance, Distance Rest
Distance, Time Rest
Time, Time Rest
Time, Distance Rest

You can also include how many reps you want to do AND if you want to include a warmup/cooldown.

I set mine up for 3min then 1min rest for 10 reps with warmup/cooldown. Basically, when you have this set up and hit start to begin your workout, you’re in “warmup” mode until you hit the LAP button. Similarly, when you finish your last rep, you will go into “cooldown.”