My coach is fond of saying that he doesn’t really care what days you do his workouts in his schedule, as long as you do them. He has also experimented greatly with varying the order of workouts to see if there is any difference in the results, and he has seen that there isn’t much difference.
Most workout schedules have a series of light workouts during the week, and then on Saturday you do a medium to long swim, a long bike, and then a short run; and on Sunday you do a long run. After the long workouts on Saturday and Sunday, you get a day of rest on Monday. I have looked at many training schedules and all of them say the same thing.
I would think that this is an attempt to maximize training benefit in performing a workout under energy depleted condition and balancing that with reduction in the chance of injury.
My first trial change was to run long the day before swimming/long bike. This didn’t seem to wipe my legs so much that I couldn’t get a decent workout on the day after running. My second change was now to run long mid-week, and whenever the sun comes out. I may attempt my long bike the same way.
Sometimes the weather just doesn’t cooperate. So at least until the rains subside in sunny California, I am now trying to build my endurance by getting outside whenever I can.