Tweek the RW plan a bit, Jinsi. They say go for 3 runs a week - you are already doing 4 runs a week. The 'M' in the schedules does mean miles.
Strides is a way of raising your heart rate by doing short sprints then dropping back down to a jog.
Try doing the RW schedule, but make your extra run a recovery run of below 70% of your working heart rate on your HRM. that way, you will do the same mileage as now, but should gain more structured training.
Heart rate and speed can't really be correlated as every one is different - example: my HR at 70% allows me to run at 7.30 pace - your HR may allow you to run faster or slower than that at the same %.
When using a new schedule, i tend to assume a time it takes me for one mile of running and so when the schedule says 'do 2M' i would take my time for one mile and double it. Does that make sense?