I have a few years on you guys (62) and know from working out for years as well as having a friend who is a Exercise/Fitness professional, that the latest research says that you get the best results by warming up slowly but do you stretching at the end of exercise or for us paddlers, after camp is set and the chores done.
Don't get me wrong, stretching after your warm-up won't hurt you any, its just that latest research has shown that it doesn't protect you from injury as the experts used to think.
A cool-down and good stretching session after a workout is the Gold standard now.
A couple of things I've learned over the years that might help people both at home and on the water.
Most people after a nights sleep, wake up, and either jump right up or after a few minutes of contemplation are up and moving. The same people if they work out would never consider jumping right into their exercise of choice without warming up. So why after 8 hrs or so of lying in a horizontal position with your blood pooling in various areas of your body dependiong on sleep position, would you just jump right up before at least loosening up/stretching a little?
Ever watch a cat? When the awake, they stretch, roll around a little bit before they start moving. We should do the same.
Every morning at home or in camp, I roll on to my back, take a deep breath, and while slowly exhaling reach my hands over my head while pointing my toes try to elongate my body as far as it will go.
Then I take a deep breath in and slowly exhale while using my hands to grab the back of my knees and very slowly bring my knees into my chest. Hold for a few easy breaths, feeling your hamstrings/vertebrate slowly release in your lower back. Exhaling, slowly extend back to your starting position. Do that a few time and you won't believe the difference when you roll out of bed/ air mattress.
On paddling trips, your upper body/arms are warmed from paddling, but your legs/lower back are tight from sitting.
When I hit the portage, I take my paddle and hold it behind my head and do a few trunk twists similar to what knowledgeable golfers do. In the same position I take a big breath in and slowly bend sideways, not bouncing, just let gravity be your guide. First one way the the other, a few times is all you need. Last but not least, I set my paddle down and taking a deep breath in, I use my hands sliding down my pants front to slowly bend forward, knees straight but not locked out and try and let my hands slide down as far to my ankles/toes/ground as I can get without forcing the stretch.
The key to doing this safely is to keep your stomach sucked in as far as possible and hold it in going down and back up.
Don't hold your breath, just breath in/exhale slowly and deeply as possible without straining.
Often what we perceive as lower back tightness, is really tightness is our hamstrings ( upper back of legs). These 2 stretches will help loosen them and in turn loosen the tightness in our lower backs from prolonged sitting or sleeping in one position all night.
Last thing that will help keep you injury free is when 2x portaging like the majority of paddler do, is take the lighter load across first

I know that may seem counter-intuitive to many, but the lighter first load will allow you to warm up your body properly, give you more flexibility if you encounter obstacle on the trail and actually make the heavier second load safer and easier to handle.
Or you could just find some nubile young portage girls that can carry 245# canoes and sign on as the cook and campfire builder

Until that happens, I don't think you'll regret trying the suggestions I mentioned.