Stretches: Soles of feet and fascia, Achilles tendons, calves, knee flexors, hamstrings, hips and lower back, elbows, shoulders.
Main Focus: Spinal length. The rest of the lengthening through the back body will come with practice.
Downward Facing Dog is a staple posture in any kind of Yoga class, often used as a place to rest and catch your breath. But for runners, it can be all the more meaningful: it can help with many common ailments, such as plantar fasciitis, calf and hamstring tightness, IT band tightness, lower back soreness, etc. It can improve your running form by decompressing the spine and correcting your posture.
But Down Dog can also be a lot more work for runners than for your average bear: while it is an excellent stretch for all of this back body tightness, practicing Down Dog can also be a really uncomfortable experience.
So follow my alignment cues here, modify to make the posture more accessible, practice it consistently, and OM to your PR. Read more for alignment cues and modifications.