Riders apologise for short sessions. "I only had forty-five minutes." The apology is usually misplaced, because forty-five minutes indoors is a different thing from a shrunken road ride, and on a wet Tuesday in January it is often the better one.
The clock is the wrong measure
Take an outdoor endurance ride of two hours. Inside it there are junctions, descents, a café, two sets of lights and a stretch where you were freewheeling down a hill, so if you add up the time you were genuinely putting power through the pedals you may find ninety minutes of actual work, much of it well below the intensity you meant to ride.
Now take forty-five minutes indoors with three ten-minute blocks in it. There is no coasting and no terrain, so every one of those thirty minutes of work happens at the intensity you chose.
This is why Load is the number worth watching rather than duration. Load comes from the intensity and the duration of the ride together, so it counts the work you actually did, and a dense indoor session and a soft outdoor one can score closer than the clock suggests. Occasionally the indoor session scores higher.
What fits in forty-five minutes
Quite a lot, as long as the session is chosen for the slot rather than crammed into it.
Sweet spot. Two blocks of fifteen minutes, or three of ten, at an effort you could just about hold a broken conversation at. This is the highest return per minute available to most riders and the reason it dominates short winter weeks.
Threshold. Three blocks of eight minutes, or two of twelve. It is harder and needs more recovery afterwards, so keep it to one session a week over winter.
VO2 max. Five or six efforts of three minutes. It is brutal in a short session because the warm-up is not optional and the recovery between efforts is long, but it does fit.
Steady endurance. Perfectly valid, and the thing most riders skip indoors because it feels unambitious. Forty-five continuous minutes in zone two with no coasting at all is a genuine aerobic stimulus, and far kinder to the rest of the week than another hard day.
What does not fit is anything that needs a long warm-up, a long main set and a proper cool-down. If a session needs ninety minutes to be itself, ride it at the weekend.
The trap of making every short session hard
Time is scarce, so every session gets loaded with intensity to make it count, and within a month you have three hard days in a five-hour week, you are not recovering between them, and your Form has been negative since the middle of December.
Intensity is not free simply because the session was short. The recovery cost of a hard forty-five minutes is closer to the cost of a hard ninety minutes than the clock suggests.
WattPlan's plan builder is deliberate about this. Below four riding days or four hours a week it puts one hard day in the week and fills the rest with aerobic work, and above that it allows two. It also treats sweet spot as costing the same recovery as threshold or VO2 work, which is why you will not see it stacked on consecutive days.
Make the warm-up real
Short sessions tempt you to skip the warm-up, and this is the one place the clock genuinely bites, because going from a desk to an eight-minute threshold effort produces a worse interval and a higher perceived cost.
Ten minutes with a couple of short lifts in it is enough, and it comes out of the session rather than being added to it. A forty-five minute slot with ten minutes of warm-up and thirty minutes of work is a real session, while forty-five minutes of intervals starting cold is a worse one.
When the short session is the right answer
Most weeks in winter, for most riders. The longer ride wins when you have the time and the weather, and there is no substitute for genuine duration when you are building towards a long event.
The rest of the time, count the sessions you completed rather than the length of the best one.