Shortening an aerobic endurance ride is never quite as beneficial as doing the whole workout. However, if you're forced to ride indoors because of bad weather, here are three things to consider:
1. If you use www.Zwift.com (virtual real-world cycling, indoors) you might find it easier to complete the longer indoor workouts. Pro riders use this training tool for similar reasons. For me it is an essential tool for triathletes during winter.
2. Riding indoors is a little harder than riding outside, because there is no momentum, freewheeling or stops. For this reason you can reduce the session by 10-15% and you'll get almost the same benefit as riding the full duration outside.
3. The most important component of cycling performance is actually your Functional Threshold Power (FTP) which can be improved by doing speed work and not just long rides. For example if you increased your FTP by 10% over a period of three months, that is like improving your endurance. Because you can now ride at the same power output as previously, but for a lower heart rate and less energy expenditure.
So it's good to ride long and don't panic if you have to shorten a workout from time to time. At the same time you do need to include long workouts on a consistent basis if you want to maximise your chances of success, particularly at Ironman 70.3 and Ironman 140.6.