not sure what you mean by your muscle mass being enough to support your BMR. your BMR has to do with all kinds of shit, one of which is muscle mass.
with respect to conditioning for the run, how far you can run comfortably now? i don't think pyramiding makes sense, i think you're probably better off doing a linear progression. that's what i'd do myself if i had six weeks to build up. here's an idea.
two days per week, with at least a full day and preferably two in between:
week 1 - run 1.5 miles, set a pace that's challenging so you're pretty gassed by the end but not dead; this becomes your pace for the next weeks
week 2 - run 2 miles, just below the pace you set in week 1
week 3 - run 2.5 miles, same pace as week 2
week 4 - run 3 miles, same pace as week 2
week 5 - run 3.5 miles, same pace
week 6 - run 4 miles one day, 5 miles the second day, same pace
three days per week:
week 1 - walk 40 minutes
week 2 - walk 40 minutes with 8 intervals of hard running, faster than your jogging pace for 30 seconds, get up on your toes
week 3 - walk 40 minutes with 10 intervals of 30 seconds
week 4 - walk 40 minutes with 8 intervals of 45 seconds
week 5 - walk 40 minutes with 10 intervals of 45 seconds
week 6 - walk 40 minutes with 8 intervals of 60 seconds