Well, the best person to ask about this would be your coach! However, I can give you some of my thoughts.
1. I would avoid "sprinkling" cross-training time across the week. Unlike mileage, the point of cross-training is solely aerobic. You get the best aerobic "bang for your buck" from fewer, longer activities. As such, a single one-hour swim or bike is better than two half-hour sessions.
2. Consider going without a complete off day. I find that a cross training day keeps me about as fresh as a total rest day, with aqua-jogging or swimming being especially gentle on the body. Removing a complete off day will make your mileage and cross-training math easier.
3. Try to preserve a non-running day in your training as long as possible. I find that I'm able to do 50-55 miles in 6 days. I think it's worth it to double once a week to protect the day off from running: I feel fresher for workouts, and it limits long-term fatigue.
The right approach for you depends on your running background. Injury history and past mileage loads are relevant here. That's why your coach can likely answer your questions better than me!