Its easy to develop strength in the hamstrings as they get shorter – think hamstring curls
There are many ways to stretch them – See best hamstring stretch
HOWEVER – being able to provide force as the knee and hip extend and the hamstrings get longer not only helps with power development it also helps to prevent against hamstring strains
Have a go at the 2 exercises below
Do 2 – 3 sets of 4 -6 reps of each prior to running or part of your warm up to see the benefits