Same as above i think really althoagh you need to do something you will enjoy as alot of peole loose interest quickly

What works for others may not work for you but the general key is like has been said above but keep pushing youself.

If you find you cannot continually jog for 20-30 mins (which is harder then you may think) try jogging for 5 mins walk for 5 then straight into a jog for another 5 and so on untill youve done 30 mins in total--then start upping your jogging time and cutting your walking time but still keeping to the 30 mins eventaully you will be jogging for 30 mins continually

Hope this may help