i used to run when i weight 240 pounds.. and out side not in the gym in the beginning your gunna be sore but youll be amazed how fast you will be able to go longer.. all you need to do is run 3 times a week! and running on the tred will be easier on your joints
good luck have fun I LOVED running then i got fatter now i cant wait to be able to run again! if you really want to getinto heres a bit of the program i followed (i was training for my ten k which i finished in 1h and 8 min)
week one
day one: run 30 sec, walk 30 sec for 15 min
day two: repeat day one
day three: repeat day one
week 2
Day four: Run 30 , walk 30 for 20 min
day five: repeat day four
day six: repeat day four
Week 3
day seven: run 1 min walk 1 min
day eight: repeat day seven
day nine: repeat day seven
and so on.. then you keep bumping it up like.. run 2 min walk 2 min
its a really really great program