I've recently started applying the 80/20 principle to my marathon training and have started running alot more in zone 2. I find however when running at this pace my calves feel like they are going to explode and become very swollen just after 2km. After about 4km my toes go numb and the discomfort causes me to speed up. As soon as I start running at a faster pace in about the zone 3/4 range, all the discomfort goes away. Any idea what this could come down to? I was thinking maybe its my form or my shoes.