I’m starting marathon training for the spring and thinking about how to structure my long runs. I want to add some long fartleks/ tempos to my schedule. When I do these, should I end the whole run with a cool down, or just the last section of whatever up tempo running I’m doing? My thought is that I want to be running fast while tired, and I lose some of that effect if I’m cooling down for miles 19 and 20 of my 20 mile run.