I set most rides by time rather than distance, because one persons 80 mile ride can take several hours longer than another persons, depending on terrain, weather, the bike they use and their power output. And people ride faster on race day than in training, so it's time in the saddle that counts most when training.
As a coach I need to set workouts that will increase your endurance for Ironman, without leaving you injured or needing to rest for several days afterwards. After all it's not what you do in one workout that counts, but what you do consistently over a period of months. Ironman 140.6 is a unique challenge because it's the one race where you can't realistically cover the actual distances in training without needing to take serious recovery time.
I also create the workouts being mindful that people have family/social responsibilities at weekends, so I try to make them time-effective. There is no great benefit to spending the entire day riding slowly.
The long rides I set for Ironman training (which progress up to 5 hours long) often include efforts at above race-pace and a run off the bike. If I made them any harder, you would not be fresh enough to train effectively for the remainder of your training that week, which includes a long run.
If you can do these long bike sessions in the middle of a tough training block, you can be very confident that after a taper period you'll be fit enough to do well at an Ironman.
However it can sometimes be good to do one or two rides by distance to help build your confidence (e.g. 100 miles), so feel free to do this with around 6 weeks to go before race day. Just make sure you consume around 100-150 calories of carbohydrate every 30 minutes to boost your glycogen levels.
NOTE ABOUT TRAINING PEAKS: You may have noticed that Training Peaks has started providing an automated distance for each bike workout, based on a speed of 14mph. I honestly don't know why they choose this speed, because it is different for everyone. Please ignore this for now, because the actual completed distance is usually further. Just stick to the Duration guidelines.