You won't maintain 100% of your fitness, but the elliptical does a fantastic job of helping you keep most of your fitness. I have had a few injuries over the past few years and always use the elliptical to either maintain my fitness or to augment my total aerobic volume when I am starting back after an injury-induced lay-off.
With respect to workouts,I find that I can get in a much higher volume (as calculated in minutes at a given heart rate) on the elliptical than I can running. Give that a test, but it would not shock me that if you did a simulation of mile repeats at tempo effort, you might be able to get in 50% more volume by time on the elliptical than you can running. This offsets some of the loss of specificity of training from doing the elliptical instead of running.
Speaking of specificity of training, I would nor recommend going with the bike over the elliptical. I do get on a stationary bike from time to time, but that is only when I cannot run at all and need to break up the monotony of extended periods on the elliptical.