Get outside and run/walk and keep it at a pace where you can hold a brief conversation (without blacking out
.) If you need to walk a bit or slow down, do just that.
Personally I find treadmills absolute murder. I can't keep my proper form and cadence on there and if run at my 10 mile or 1/2 marathon pace (9.30/45 per mile) I can only last about 30-40 mins, whereas outdoors with subtle variations in pace, cadence (leg speed) and incline I can run at this pace for 2 hours plus.
Just over 2 years ago I was lugging around over 21st (I'm 5'9) and healthwise was an absolute disaster zone.
I'm now just over 14st, have joined my local club, done 30 races so far this year on and off road with my longest so far being 25k and have just set four PB's in 15 days (10k/10mile/half marathon/club handicap (3.5 miles)).
I'm 48 and have been away from any form of regular excercise for the best part of 10 years. I'm no racing snake and no real athlete but I get an absolute blast from doing it and as the old saying goes if I can, ANYONE can
Get out there and enjoy yourself 
(and don't use a freakin' treadmill
)
Edited: 03/12/2012 at 21:13