You might have known this line was coming but it's too late to worry about diminishing returns. You experience diminishing returns as soon as you take your second step.
Assuming you're really worried about no returns or, worse, negative returns, I wouldn't be worried about that yet. My suggestion would be the same as it always is for building a training plan. Pick your next goal race (maybe a spring 5K attempt) and work back from there. If the question is what to do as you increase your intensity, I'd say maintain as much volume as possible while making sure your workouts don't suffer. If you can maintain volume, great. If not, don't worry if it falls back a bit.
If your question is whether or not you should begin increasing the intensity some, I'd say go for it to some extent. Thanks in large part to the words of denton and Andrew, as well as my reflections on my own personal experiences, I'm becoming a strong believer in never getting too far away from speed. If you're doing nothing but 80+ miles a week at slow paces, maybe it's time to drop it back to 75+ with some strides or tempo runs thrown in. Better yet, if you're now throwing up triple digit weeks of all slow running, stay in the 80+ range, maybe even stay in the triple digits if you can, while throwing in some faster work.
Don't be surprised if your volume naturally falls back some as you increase your intensity. However, let your body's response dictate the extent of that. Don't go in planning to decrease by X amount.
