depending on your age and fitness level and genetics cycling can help in different ways. I'm in my 40s, ran a lot in my teens and 20s then switched to cycling for about 10 years, then got back into running to train for marathons but always broke down near 20, due to shin fracture issues for 3 years straight. Taking is slower this year for a spring marathon, but more than half of my endurance workout will be on the bike. I had no trouble running 30+ miles in my 20s, but age and genetics is catching up to me. Anytime you can raise your heart rate to an endurance level it will help, I plan on only running one long run every 10 days.