My experience is similar to yours - although it should be easier to run a given pace on a treadmill, I find treadmill running pretty hard. I believe it's the boredom - the mental aspect of running is pretty strong, eh? As others have said, it does get easier, especially if you use the treadmill for faster, shorter bursts, or intervals.
Mind you, one of the staff att my local gym ran a marathon on the treadmill for charity - must have an incredibly high boredom threshold!
I'm 55 and in the 'fitter than my kids and often taken for 10 years younger" group. However, and this may be off topic, I'm troubled of late by the repeated appearances of some old bald bloke in all our family photos...
I love the guy. Anyone who can force James Naughty into committing an embarrassing semi-spoonerism live on air and then getting a fit of the giggles gets my vote. I notice the BBC always say 'The culture secretary, Jeremy Hunt' now, presumably to avoid any further involuntary profanities over the airwaves.