I am 67 and just dropping below 98k, chronic lower back, knee brace, 2 blown out rotator cuffs....issues, but nothing too bad.
Variety of SUP disciplines helps me get by.
I am a lazy b'tard. I won't do any exercise for more than 3 weeks. I simply get bored and tail off.
So my only fitness comes from passion. I love SUP surfing. I love paddling my Surf SUP's on flat-water. I love Golf.
I may ride a short SUP at dawn for 3 hours, eat lunch, nap, then ride 10'6 SUP for a few hours in the afternoon. (Stable SUP's add water time and give tired muscles a rest, while adding to conditioning)
Or I paddle flat-water for 6 to 10 k on my surf SUP's in the afternoon
Yesterday I felt the need, so I pushed hard for 8k on my 10'6 Surf SUP... burned my whole body and lungs.... felt great! (I'll go slower today)
When I can afford it I love the 9 k walk that comes automatically with a round of golf. Walking itself bores me.... with golf... it's included. Plus the occasional swing, works wonders to unkink my lower back.
In summer, I have no problem surfing 5-6 days a week with 3+ hour sessions average.... but conditions are calmer, waves small and challenge is less.
Passion keeps me in the water, not conditioning

My goal's
Winter: 10 hours a week on SUP
The rest: 20 hours a week on SUP
I come close.