Wow, that is a tremendous gain. 7 minutes! I have always referred to the treadmill as dreadmill because that's how much I despise it. Lol. It's my opinion it causes you to have an unnatural gait pattern since it doesn't have all the divots and unevenly terrain you'd encounter outdoors.
But, it can allow you to push yourself in ways you can't outside. I just relocated to Minnesota & I heard the winters here are BAD plus i'm an asthmatic who's usually triggered by cold air so i may be on the old dreadmill more frequently this winter. Ill have to remember your GAIN and be inspired =)
I think the secret to the treadmill is to have a focus, something to specifically train for a session, not just running to clock up the miles. That would bore me to tears.
I find that running on the treadmill is actually easier on my muscles and joints because it is a nice flat and forgiving surface. Running on the roads or trails here is always a risky thing to do (especially in the dark with no torch!) because they are so rough.
Most of the trails I train on is old and crumbly reef rock, so it is really hard on your ankles.
I am happy to be back running outside though ;-)