HIIT vs 4 hour runs

It probably depends what you are doing on the 4 hour rides. Most people who race will do a mixture of longer stuff and high intensity - the mix will depend on time available and the aim. The right mix is a matter for debate of course but yes I know people who have gone into long distance cycling and upped their mileage but found they lose a bit of top end.

https://www.trainingpeaks.com/blog/3-workouts-to-raise-your-functional-threshold-power/

A mix of intervals and a longer ride. I'm not sure if the intervals suggested in the article count as HIIT as they are probably too long. Also, what is your aim? Is it purely to have greater power over an hour or are you looking at a general improvement in your cycling? If all you are looking at doing is increasing power to race a 10 or 25 mile TT then it may be that the 4 hour rides become less important.

These are great, especially to get started before higher intensities if haven't already been sprinkling in higher intensities into endurance rides.

Pross wrote:

https://www.trainingpeaks.com/blog/3-workouts-to-raise-your-functional-threshold-power/

A mix of intervals and a longer ride. I'm not sure if the intervals suggested in the article count as HIIT as they are probably too long. Also, what is your aim? Is it purely to have greater power over an hour or are you looking at a general improvement in your cycling? If all you are looking at doing is increasing power to race a 10 or 25 mile TT then it may be that the 4 hour rides become less important.