I'm new to running. How much should I train at my MAF HR?

It’s not how many miles you run, but how much time you spend working out. If you haven’t run a lot before (or done so sporadically), bookend a 10-15 minute MAF HR workout with 15 minutes of warm-up and cool-down. Start your warm-up 20-25 BPM below MAF and slowly and evenly ramp up so that you reach your MAF HR at the end of the 15 minutes. For the cool-down, ramp your heart rate down slowly and evenly so that you return to 20-25 BPM below MAF by the end of your workout.

Start by doing this every other day (3-4 days a week). If after 2 weeks you feel like you can do more, increase your time at the MAF HR by 5 minutes.