At some point in the next few weeks you will do your last comfortable outdoor evening ride of the year. The switch indoors is worth treating as a deliberate change rather than something that happens to you when it gets dark.
Set the space up once, properly
Almost every complaint about indoor training traces back to the setup rather than the training.
Buy the fan first. It does more for whether you finish a session than anything else you own. Outdoors, moving air carries heat off you constantly, and indoors there is none, so your core temperature climbs until your body starts protecting itself by cutting your power. Aim a large fan at your chest and face.
Something to catch sweat. Sweat is corrosive and it lands on your stem, headset and top tube, so a towel over the bars and a mat underneath will save you a bill.
A drink within reach, already made. You will get through considerably more per hour indoors than out.
The bike already on the trainer. The friction of setting up is what kills a Tuesday session, because if riding costs ten minutes of preparation before you can start, you will skip it on exactly the evenings that matter most.
Expect the numbers to change
This catches riders out every autumn, when the same effort produces less power indoors and the same power feels harder. Heat is the main reason, with position and the absence of freewheeling behind it.
You have not lost fitness and your outdoor numbers were not wrong. The two environments differ enough that comparing single rides across them tells you very little. There is more on this in why your indoor power looks lower, including what to do about your FTP.
What you can compare is indoors against indoors, week on week, and that comparison is clean because the conditions barely change.
Rebuild the week around what the turbo is good at
Three hours indoors is an endurance feat in a sense nobody intended, and most riders cannot sustain the habit, so long unstructured riding is where the turbo is at its worst.
Precision is where it wins. There are no junctions, no descents, no traffic lights and no terrain, so if a session asks for eight minutes at a given effort, you get eight minutes at that effort in every interval, exactly. Outdoors you almost never do.
So rather than simply moving your outdoor rides inside, put the structured work indoors where structure is easy, and keep whatever long steady riding you can manage outdoors at the weekend, where duration is bearable. When the weekend is unrideable, a shorter indoor ride with some tempo in it beats a grim three-hour attempt at replicating the real thing.
Shorten the sessions and accept it
An hour indoors is not an hour outdoors. There is no coasting, no descending and no junctions, so the pedalling is genuinely continuous, and a 60-minute indoor session can contain more work than a 90-minute ride on the road.
WattPlan handles this by scoring what you actually did. Load comes from the intensity and duration of the ride itself, so a dense indoor hour and a soft outdoor two hours are compared on the work done rather than on the clock.
Give yourself something to look at
Whether it is a screen, a series, music or a virtual world, the riders still going in February are usually the ones who stopped pretending they should not need it.
One caveat. If a session has real intervals in it, the entertainment has to be something you can ignore for eight minutes at a time, so save whatever needs your attention for the steady rides.
The first fortnight will be the worst
The first few sessions feel disproportionately hard, you sweat more than seems reasonable and your power looks disappointing. Heat tolerance indoors is itself trainable, and most riders adapt within two weeks.
Judge the whole winter on that first week and you will conclude you have lost fitness you have not lost. Give it a fortnight before drawing any conclusion, and let the chart rather than a single ride tell you what is happening.