I'm no expert other than about what I learned at the time of two treadmill tests. I don't think that maximum heart rate has much to do with MET level, other than the fact that you will probably hit your own personal maximum heart rate when you max out the METs. A treadmill test normally uses a scale called the Bruce Protocol. The score in METs is calculated from the treadmill speed, angle of inclination, and maybe some other factors like your body weight.
Supposedly, if you are slow walking at 2mph, that is 2 METs. If you are fast walking at 4 mph, that is 4 METs. The doctors want to see everyone capable of 5 METs, just to get through the stresses of modern life. To be capable of doing light mountaineering (whatever that is), you should do 8-9 METs. A young, athletic, adult male is typically capable of 12 METs. If you are Lance Armstrong or otherwise a super endurance athlete, you might be capable of 16-18 METs.
In two tests, I had scores of 13.8 and 14.3. That's completely amazing to me, because I'm certainly no super endurance anything. Maybe super couch potato.
For a first approximation, your theoretical maximum heart rate is estimated at 220 minus your age. So, for a 30-year-old, that would suggest 190 max. They like to see you knocking out a high level of exercise with a moderate heart rate, like say 14 METs on a heart rate of 130. That would suggest that you have developed a high cardiac stroke volume typical of an endurance athlete. During my treadmill tests, I was doing 102% of the estimated max heart rate. OK, that simply means that I can push myself to the limit. Psychologically, many people cannot.
If you are a distance runner, then maybe you have reduced your rest heart rate to 45 or 50. That is a good sign. It is rare to see a distance runner with a high rest heart rate like 70 unless there is a thyroid problem.
For an endurance hiker, you probably don't need much of those extremes once you are away from the treadmill. Instead, you probably strive for efficient athletic output over a lot of hours. For example, you want to be able to backpack 20 miles with a 20-pound pack in 8 hours, and do that day after day on a 3500 calorie diet (or something like that).
So, if you are interested, get your physician or a cardiologist to order a treadmill test. Once you have results from that, they can tell you if it is normal or what to do next. You'll probably need a sports medicine doctor to really take you to the next level.
Personally, I get a little more interested in the performance of Ed Viesturs. When most people do their hiking at higher elevations where the air is thin, the rest heart rate increases, and the maximal heart rate decreases. That continues until such an elevation where those two lines intersect, and that is your ceiling. Ed kind of keeps on going, and he has been up Everest several times without supplemental oxygen. Now, sure, he stays in good shape by running, and that gives him room at the bottom end of rest heart rate, but somehow that line never quite intersects with the maximal line. I would even settle for one Everest in the bag.