Any aerobic exercise that uses your legs will help your cycling. But what helps your cycling the most is ............................ cycling. Running will help some and give a nice change of pace. But forms of atheletic endeavor do not translate that well from one to another. Lance Armstrong, a great cyclist, ran a decent marathon. Michael Jordan, a basketball great, was a passable minor league ball player. Do you get the idea?

Also, running being both high impact and weight bearing has much higher injury rates than cycling. So if you do very much of it, the risk you are taking in injury would possibly out weigh the cross training effect. Walking at high speed up a steep incline might be better cross training. It has much less injury risk with virtually all of the benefits.