"The best and simplest way to get a high level of fitness is doing repeat 100 yard sprints," says Gavin. Unless you are a conditioned athlete, Gavin recommends running them at about 80 percent of your possible intensity. After completing one, you have a 75-second window to jog back and get ready for the next. In this way, you move between your anaerobic and aerobic systems. Do about 8 to 10 repetitions.