The right balance of circuit training and cardio?

say_pocket

Right now, my goals are more muscle building and "toning" related. I do 30 Day Shred three times a week, P90x3 MMX or Warrior once a week, P90x Yoga once a week, and I see a personal trainer once a week. So, five days of hard exercise, one day of yoga, and one day of rest. It's been working out well for what I want and I feel like I'm seeing results.

But now I have some friends who are looking to do a couple fun 5Ks this summer. I've never been a runner. Or even a jogger. (Unless the bus is rolling up to my stop and I don't want to miss it. Ha.) I feel like I should add some running training to my routine to build myself up to being able to do 3 miles without dying. Is it a terrible idea to stack some treadmill time on top of what I'm already doing? Is that too much? Do I have to cut out one of my usual work outs to do this right?

On top of all of that, I have a couple overly optimistic friends who want to ride the RAGBRAI in 2015. Last summer, the longest distance I biked was 10 miles and it killed me. So I told them I'd try to start riding more and see if I can't train to up my endurance in that as well. If I replace one circuit training day with some endurance bike riding, will that help with my running endurance? I know they're different things and different muscles, but is cardio endurance the same all around? Any runners or cyclists on here with an opinion?