These runs should be done at a comfortable pace. For those who use heart monitors, run between 65 to 75 percent of maximum. In other words, run easy.
A continuous run with a buildup in the middle to near race pace. Begin with 10-15 minutes easy running, build to 10-20 minutes near the middle, then 5-10 minutes easy toward the end. The pace buildup should be gradual, not sudden.
Run the 400s at about the pace you would run in a 5K race. Walk or jog between each repeat. Best done on a 400-meter track, but you can also run this workout on the road or trails.
This could be swimming, cycling, walking, or other forms of aerobic training. Cross-training days should be considered easy days that allow you to recover from the hard running you do the rest of the week.
Rest is an important part of your training. Be realistic about your fatigue level and don't feel guilty if you decide to take an additional day off.