Hey,
I started running last summer (trained for the 5k race for life), built up the running until I could run for 10 mins then just gradually made the runs longer...did the 5k in 33 mins. Then my running buddy went off to uni, so I stopped. Started again this year and entered the 10k race for life, managed it in 1 hour 5 mins!!! I'm not a natural runner, I haven't got proper shoes yet, but am trying!! I enjoy it like you, but it is tough!
I think for me its the type of exercise that really gets my heart rate up if I incorporate some sprints. I also try circuit runs - sprinting to certain point then stopping and doing lunges, sprinting again doing squats, etc etc that really burns!!!
I would say just build up the distance gradually, don't worry about pace yet! And enter a 5k race, as it really gives you the motivation to run! Make your first aim running the whole way round?
I don't think it makes a difference compared with other exercises, maybe I use more muscles when running than on a cross trainer in the gym, but I still do strength training. They say for every mile you run, you burn 100 calories....i think that's the rough guide anyway!