it's hard to do it on a treadmill, but i recommend running faster for 2 min, rest 30 sec, and do it again until you have 10 min, keep raising the number of repetitions each week until you get to 30min.
The thing with running outside is that it can stabilize the way you run. Some people do not know how to run efficiently, so if you run at all you can give up a hill your body takes the right position, if you run down hill too, so your body mechanics better themselves.
Also running outside you have sidewakls, rocks, things that break your breathing and thus tire you faster. Remember hit is not only sprints (like you would like to do on a treadmill) outside you can touch the floor every 30 sec, or jump with your knees up 10 times after each spring of 90 sec (going all you can run 90 sec is good) then jog don't walk, jog for 1 minute and repeat. You can do al sorts of stuff outside that make HIT stay loyal to its purpose.
hope this helps.