It might be because I swam a bit, first, but I really didn't do much more than familiarise myself with the snorkel I've bought to eliminate proper breathing from the swimming equation.
Luckily for me, the route I chose was pretty flat, and had quite a lot of downhill sections. It had a few hills, of course, but they were tiny, and not steep or high. I just never got into a nice groove. I thought maybe I was just having a rough start, but the rough start turned into a rough middle, and finally a rasping end.
We all have bad runs. We all have bad spins if we spin, too. The idea I thought of as all this was happening is that maybe your "body" (which turns out to really be just a part of your mind) sometimes tests your mind, to see if it can be talked into taking things a bit easier. All sorts of parts complain and get prematurely tired just to give you that excuse to stop. So I decided that this is a good opportunity to just say NO to that. Hopefully next time the body remembers that The Answer is No from the mind. No, we're not going to find a comfy chair, we're going to push on.
Just one other thing. Although I doubt that it measures things with the right kind of accuracy, the instrumentation on the spinning cycle is probably at least consistent. So if it gives you one reading on one day, and then a different one on another, the relationship between the two is at least a good representation of the facts. So the other day I think I almost did 450 calories and today I did 390. Looks like the perception of the "tough run" is quite true then.