Dump lots of oatmeal down the filler pipe.  Improves overall performance, but might result in increased emissions.  Good luck 

Another possibility is to mount the engine in an easier to drive hull. Or, a hull big enough for 2 engines and mount a smaller, newer model as auxiliary propulsion. Second solution works really well when it's a plug compatible model, but as with any 2-engine installation your maintenance doubles and upgrades get really complicated, even if the engine mounts are compatible.

Laszlo

An upgrade with an auxillary.....  I like it!  Maybe one of those new Swedish models, eh?  Hope I don't blow a gasket - I hear those Swedish models take a toll when matched up with some of the old hardware putzing around out there.  Have to check with the Boss.

Barring an auxilliary engine, especially the swedish type, I'd suggest cranking the old engine a little more often during the "off" season.  Take it out for a spin with some other bodies during the cold months.  Maybe take it to a pool and crank it up there.  

And because I can't think of any more metaphors for cars and engines (I'm thick sometimes) I'll say it in direct terms.  Cross training is a fantastic thing.  Get out for walks/runs or bike rides whenever you can.  It doesn't have to be anything too rigorous, just routine.  As for joints, regular exercise will help those too.  Swimming will be the best thing to keep your shoulders in good shape for paddling, and might I suggest, next time start with a slightly shorter first trip out for the season.  15 miles doesn't seem like much, but I usually do just a few miles my first time out and work up to the 15 milers.  YMMV and all that, of course.

FrankP 

15 miles return!!! most young athletes wouldn't contemplate that! Look, try a different means of propulsion such as rowing. I've rigged a 16' canoe for rowing and it is just so light and sweet on the water. This offers much greater efficiency and more choice of  ergonomic positions and you just glide glide glide! Wear a bike helmet with a rear vision mirror to see where you are going if you like. sliding seat rowing is even better and takes the load away from your lower back if that could be a problem. I would recommend oars about 8'6"  with appropriate riggers to give maximum sweep for minimum load. You will find them surprisingly light. Maybe a MC 16 with a drop in pedal system like spin fin is an option.  All this would assist the cross training idea too. Take your joints through a gentle full range of movement every day, along with gentle sustained stretches as this improves the circulation and nutrition to connective tissue notorious for poor blood supply. Get someone to give you some deep finger massage into the area about a joint, called "deep tendon friction" and this will work marvels. Soaking in a hot tub helps as well. take heart, if you can do 15 miles return without training, you're still a spring chicken.