Now that you have your MHR, you can increase the number of days/week that you run. Run hard (up to but not over 158 3x/week) and run easy AND a shorter distance 3x/week. Your recovery HR should be between 118 and 140, but I'd suggest you set your alarm for 138 or even a couple of beats lower on these easy days.
If your normal run is 6 miles, run only 3 miles on these recovery days.
Running training should generally follow a pattern of hard day, easy day, hard day, easy day, and so on with an occasional day of rest from running... usually following a hard day. Long runs, tempos, and intervals count as hard days. They should be followed by a recovery short run day at easy pace or, once a week or so, a complete day off.
As Coach JS says, it's possible to train well and run only three days a week, but since you don't have a coach, the hard, easy, hard, easy pattern is pretty easy to implement. Good luck.