Me and my mum built up the same sort of way, we jogged for the distance between 4 lampposts and then walked for 2, had a day off then repeated until we'd done that set up 3 times then increased jogging to 5 lampposts, did that on alternate days etc. that was last year and we kind of stopped going so having to start all over again now! If you miss just one or two jogging sessions you can really tell the difference, you slip back in how well you were doing much quicker than you can progress so just keep going with it! Also, my PE teacher told me to breathe through my nose as it stops any chest pains! Oh, and a sports bra is essential!!