Strides are basically a 10s-30s segment where you start from a standing position, accelerate up to 5k pace or mile pace, and then bring it back down. The objective is to focus on form (proper foot strike, posture, very high cadence) while running near top end speed, thus improving running economy. These are usually done at the end of an easier session. Walking back allows for adequate recovery (the point is to improve technique, which is much harder when you’re anaerobic/fatigued). For 10s strides, I usually accelerate (up to 1mi/5k pace) for 4s, hold that for 2s, and decelerate for 4s.