What many people that run and/or cycle don't realize is the two activities use mostly different sets of muscles. Cycling uses chiefly the quads and hip flexors while running uses the calves and hamstrings. I've done a lot of both and hit 2600 miles one year in running in between a dozen years of 2000+ miles. I never really felt a carry over from one sport to the other. Running won't really detract from cycling and vice versa if you do it just for fitness. However if you have limited time and energy, one will suffer. Also it's not possible to perform at peak level doing both.