Running accounts for 70-80% of all triathlon-related injuries. Most arise through lack of strength, poor technique or inappropriate training volume. When you run long fatigue is inevitable, and even a good technique can fall apart. Your gait can alter, starting a whole chain of events that can lead to injury.
Each stride has an impact, which causes muscle soreness and fatigue. Longer runs require longer recovery, and this can have a negative impact on subsequent training sessions.
An alternative strategy would be:
Plan one long run per month followed by two easy days for recovery. Low intensity cycling and/or swimming are good. Add 10-15mins to each long run, peaking at around 2:30hrs (use duration rather than mileage to account for differing terrain). This will build confidence in your run endurance.
On other weeks, complete ‘double run’ days instead of the long run. A good option is to run for 45-75 mins at an easy pace in the morning (maybe at your IM race pace to work on biomechanical efficiency). Your second run will then be a much higher tempo effort of 45-60mins to build muscular endurance at half marathon/10k pace.
The ‘double run’ will allow you to train without compromising technique due to fatigue, but at the same time allows you to complete a bigger distance on the same day. Several hours of recovery between runs will allow you to refuel and give the muscles partial recovery.