Should I only train at my MAF heart rate?

No. It’s important to develop the aerobic system by training at the MAF heart rate exclusively for a period of 3-6 months before incorporating anaerobic (strength/power) training into your exercise routine.

If you’re chronically overtrained, injured or ill, however, it becomes much more important for you to train exclusively at your MAF heart rate (while incorporating a proper 15-minute warm-ups and cool-down) until you are healthy, even if it takes more than 6 months.