Cycling during marathon training

Check out the FIRST marathon plan. 3 quality runs a week, and cross training 3 days a week. It works well for some people. I am on my first cycle after three previous marathons using the Higdon plans. I am using this to build for a fall marathon, and still race an Olympic tri in Aug.

Short answer is no, cycling doesn't substitute for running. So if the plan calls for five days running and you cut some of those runs in favour of cycling, you won't get all the benefits of the plan. But as the previous poster said, there are plans (like FIRST, and Matt Fitzgerald has some too) that will have you running less days and incorporating more other activities.

I personally think that if you truly want to reach your marathon potential, you would have to focus on just running. But for many people that's not practical or desirable. You can do very well on plans that combine running with other activities.