treadmills are incredibly boring, but I use one at gym once in a while , need to be aware though that its easier than running outside (no wind resistance etc) and you gait will be slightly different also , as others have said you will benefit more by running outside , my eldest keeps asking if we can get a treadmill but I haven't buckled yet
I was told on a treadmill you basically bounce along rather than run properly. I probably do a quarter of my running on a treadmill and I can get off after 10k and feel fine and fairly fresh if a little bored. 4k outside and I'm blowing out my arse normally.