As a result I've recently set a personal goal to repeat my (solo) efforts by doing a 20 klms + paddle at least once a month.
I've tried trolling on these long paddles without success, so I've decided to focus on just paddling on these particular trips.
Also I wear a Garmin Heart Rate (HR) and GPS watch and manage to keep the old ticker running at 75% to 100% of maximum beats per minute (for my age) during the entire time.
P.S. My current age at the start of this post is 61 years young....

Below are the results of my last two (approx) half marathons.
Hope this inspires other motivated paddlers to do similar.
Some stats:
Date............Distance.......Average Speed.......Max Speed............Time.............Area Type
29 July..........22.20 klms......6.0 klms..............12.0 klms...........3hrs 43min.......River & Ocean
31 August......20.23 klms......5.9 klms..............10.7 klms...........3hrs 27min...........Ocean