If your client is training for a marathon, I agree with Harris and would have them stick to outdoor running. Part of the training is adapting to the elements they’ll be dealing with at the actual race – temperature, wind, running surface, etc. I would only recommend training on a treadmill in the case of extremely bad weather, but only as a last resort. I wouldn’t bother with the elliptical for marathon training – that’s for older people with weathered joints like me!
I found that distances on elliptical trainers and treadmills cannot easily be compared. They are sometimes not even the same between different manufacturers. If she really wants to equate the two, I would go about it a round about way. Check her heart rate on both machines to see whether the effort is about the same and then see what distances she covers for 10 minutes on either equipment. Then you will have an approximation how those different machine readings compare to one another.
Other than that, I second Harris’ and Paul’s comments (even though I take exception with the elliptical being for older people – good thing he only spoke for himself ;-).
Thank you for your comments. My client is training for a marathon outside of my training with her. I do completely understand that running is the best way for training for a marathon, this is her 4th. I have train for half marathons before. However she just asked me this question, and I didn’t know where to find the a real answer instead of doing the same thing she did, Goggling it. Karin, Thank you, I will do that and forward the information to her. Thanks again Karin, Paul, and Harris.