Sounds like the perfect combination to me! If you want to complete your city you will have to plan and run every street anyway, so combining it with something that requires high mileage will just get you there faster 
If your training plan has set distances you could even plan all your runs in one sitting and then just grab whatever route you need for a particular workout (this would help: Show planned routes in lifemap/cityview !). I’m not on a training plan, but I usually plan a week’s worth of routes in advance and load them on my watch, it doesn’t take that long. The hard part is probably actually finding the time for both marathon training mileage and CS overheads from having to get somewhere before you can even start your run.