Cardio, is SUP enough?

[Eagle]

That age related calc is way off for us.  If we followed that we would be near 30 years younger at max HR.  Our average HR just doing a moderately fast hike uphill is already at 100% based on that calc.  We understand that you do need the mask to get an accurate calc of your actual VO2 max.

[eastbound]

downhill mountain running makes my knees and hips ache just considering....

[Eagle]

Yeah - before switching to barefoot forefoot midfoot striking we could not even stand up after running down a hill.  The heel strike Nike waffle caused many problems.  Just try running across the room barefoot and see how your foot lands.   

[TallDude]

I'm 53 and paddle 5 to 8 miles 3 to 4 times a week. No gym. No shoulder, or back pain. I will get back and shoulder pain when I SupSurf because I'm trying get my 1/2 sunk board moving and I use a shorter, stiffer, bigger surface area paddle. Actually, most of my sup related injuries come from surfing. As well as board damage. I have plantar fasciitis issues too. That's part of the getting old cycle no matter what sport you're into. I know a girl who wears running shoes with orthotics on her SUP. She says it helps reduce the PF pain.
Your X-14 is a great board for racing and exercise. You should invest in a really good paddle to. As was said, a smaller blade (85 to 95) and a softer flex paddle shaft well save your shoulders. It should be 8 to 10 inches over your head so you can drive it deep into the wind. My shorter paddle is about 2" over my head, and I get back pain from it. My daily paddle is 50% into a 10 to 20 mph head wind. A mix of open ocean and flat water harbors. A long paddle really helps me in the wind. 

[SUPdw]

One way I have found to raise my HR is to stagger my feet by 12 - 18 inches.  Bends the knees, and gets the legs more involved.  I switch back and forth between reg and goofy.  May not be enough cardio, but should raise your HR.