Excellent posts above, thanks all. I could write pages on this having similar experience struggling compared to recent times. Come from a similar competition background as Impala including athletics, rugby, triathlon (age group champion at one point, Ironman), open water swimming (Rottnest Channel Swim 20kms solo and duos - 6hr best solo time in my mid-fifties), cycling, multiple Avon Descents (downriver 135km two day race) and over last few years downwind paddling.
I write the above not to big-note but for context - I've done a fair bit of stuff continuously over the years. I've just turned 66 and about three year ago started struggling with breathing, erratic heart-rate, poor racing performances off the mark, dizzy spells on the bike etc. Yes, ended up diagnosed with developing atherosclerosis and PVC - premature ventricular contraction - where heart beats erratically under effort esp above 75% Upshot of the latter is heart efficiency diminished significantly and randomly, some days better than others, some parts - generally last two thirds of an event less trying than first third. BTW non-life-threatening, I'm not going to cark it or anything as a result of exercise just diminished exercise performance.
My point in regard to above not to say you have a heart condition (worth getting checked out anyway) is that bodily changes can take place over time so that when you hit a high-intensity mode for an extended period your body (strength, stamina, heart, lactic build-up etc) can't keep up as it used also when potentially exacerbated by maybe one or two significant things as with me. Just getting old in part plus some potential physical malfunction. This also beyond the physical in terms of fitness - my resting heart rate atm 52bpm. (Have masses of HR data which is very interesting in terms of recovery but irrelevant here.)
Consequences other than the physical I find hardest to deal with, where previously able to match it pretty much with mates in whatever discipline who are 15-20 years younger; now I'm struggling and it pisses me big-time. (On medication etc but no silver bullet.) Throw in for me late onset asthma - WTF! - which I can manage, mostly only comes along occasionally - and the psychological domain becomes the challenge.
Some solutions.
1. Do not relent! F*ck it, just do the best you can and get on with what you enjoy doing to the best of your ability. Impossible to deny ego-dentingness of it all, however in the greater picture let the Zen prevail and just deal with it. Rule #5 (
HTFUHTFU)
2. As advice above - and I love the comments about amount of intensity required in shorter chop even when its big - pace yourself. Make the most of rest-time available. As David Mocke pointed out when I had a three-on-one training session with him when he was in Perth for The Doctor, which he demonstrated by clapping his hands, maintain your cadence but vary your effort within that. Imagine soft claps and loud claps but same pace.
3. Modify kit accordingly. Shorten paddle length and reduce blade size and paddle weight. For me boat weight counts - I reckon a lighter boat gets up and going more easily, ie less effort to get to same speed.
4. Modify and/or improve technique. Looking at my own GoPro bow footage my own paddling needs heaps of work using all body-bits effectively. For me much much more with the legs and better rotation. (These I'll be focusing on in two sessions a week training up for Mauritius this year). Just use arms for example and pretty soon your blowing like a bellows yourself.
5. Strength makes a difference, so though frustratingly guppy-mouthed for a good part of every downwinder, reckon that for me at least, dedicated strength training will mean less effort required as it were to go hard. Same as strength training on the bike, cadence one thing but strength and specific training of it, altogether another.
6. As mentioned paddling smart makes a big difference, linking runs etc. This easier said than done for me when getting up to and maintaining speed required to hold runs finds me gasping for breath - difficult to sustain.
Sorry about the length of response but was good for me anyway just to unload. Best of luck and hang in there. Can't resist - this from Tennyson's "Ulysses":
Though much is taken, much abides; and though
We are not now that strength which in old days
Moved earth and heaven, that which we are, we are,
One equal temper of heroic hearts,
Made weak by time and fate, but strong in will
To strive, to seek, to find, and not to yield.