Quite often the question is: Is it better to train indoors, or out? The answer isn't always apparent and there are a few things to consider.
Training for endurance sports is -for most of us- a balancing act. Work, family, study, socialising, travel or other sports are just some of the things we need to take into consideration. It would be 'unsustainable' to attempt to focus too much on one at the expense of one or more of the others.
There is a succinct benefit to training indoors when time becomes crunched.
Invariably, we can be more time-efficient indoors, particularly with cycling. No coasting, no stopping, no missed turns or misdirection: time spent on the bike indoors is spent pedaling!
As mentioned, time spent indoors is always spent well. As a result, the quality of the training is usually outstanding. Completing sets of intervals whether it be in the pool, treadmill or bike trainer is easier - no time spent coasting during the middle of a bike interval or time lost stopping to cross the road during a run effort.
This isn't to say that you cannot achieve high-quality sets whilst outdoors, simply that there are more variables -and often barriers- to consider.
Motivation & Psychological Factors:
Indoor training can be psychologically tough. It isn't easy spending a lot of time indoors.
The nature of endurance sports does mean that time spent training (in one form or another) has a proportional correlation with fitness. Some sessions, admittedly, are probably not ideal for most athletes to complete indoors. Longer bike or run sessions are ideal examples.
In my experience, 90 minutes tends to be the 'limit' for most athletes (especially if they are completing it by themselves). Visualisation can become a key motivational factor to help see athletes through this. In recent times, technology has become available to help athletes break through this barrier through 'virtual' riding and running.
Some athletes, however, have no problem completing longer sets indoors without it becoming a 'drag' as they relish the opportunity to embrace the session and the benefits. Reaching milestones whilst training predominantly indoors can be a huge motivation.
Training outdoors is -for most athletes- psychologically optimal.
Humans are, after all, meant to be outdoors. The opportunity to connect with nature and be outside can be particularly beneficial for athletes' wellbeing and intrinsic motivation to train.
Personally, I relish opportunities to run on a quiet trail or wind through a valley on my bike as I find it meditative and stress-releasing. This isn't uncommon and is backed up by a lot of research.
It really comes down to how you feel, which may indeed change day-to-day.
It is important to consider the skill component of any sport whilst in pursuit of its mastery.
Training indoors, whilst often optimal from a quality standpoint, doesn't always provide you with the opportunity to develop your skills.
Depending on the athlete, this can be a vital component of their development.
In the case of a triathlete, open-water swimming is vital to improve skills such as sighting, turning, water entries and exits, drafting and 'reading' the water.
Likewise, indoor cycling might make you incredibly fit but won't help you 'ride'. Any cyclist who has completed a criterium race, for example, will testify that no matter how fit you are, if your skills aren't up to scratch you'll be left behind pretty quickly!
On the other hand, there are some skills which can be developed through focused indoor training, such as riding on rollers, for example, which can vastly improve an athletes' balance on the bike.
Overall, it's vitally important for you and/or your coach to be able to critically examine your skillsets and needs as an athlete to help you determine how you will approach your training.