As quite a severe overpronator, too much running usually bring injuries and niggles, so I do quite a lot of cycling to improve my fitness. I've found that when i've been injured this has allowed me to pretty much maintain my long running endurance. I tend to substitute time I would have spent running for about the same plus 10%. So for a two hour run i'd cycle 2.15.
I have found that I've had to be more vigilant to stretching though as if I slack then my legs do tighten up.
Personally, I have troubles with cross-training machines in gyms, as they seem to do more damage than good for me, making my shins "click" because they force the foot into an unnatural position for me, but I haven't spoken to anyone else who has had this problem with them.