Jim...Bill Dellinger suggests 10-12 mile runs at half-marathon to marathon race pace during his pre-competition phase. I have used them before and enjoy them; I also believe they help me develop fitness a little differently/better than T-pace runs. Plus, it's just an option for variety.
I used a little convoluted figuring but it worked out. Anyway, I'll use an example. Let's say a guy running 20:00 for 4 miles. His tempo run pace (for the same 4 miles) should be 5:20's, which is 21:20. So, I then played around with marathon pace (2:26:45) to figure out pace...5:35.8.
Next, I kept putting in numbers to see what distance at this pace would be equivalent to the 4-mile tempo run. In this case, 10 miles would be 56:56...5:41 pace. So, 10 miles would be slightly slower than marathon pace. 9 miles at equivalent VDot would be 50:54 (5:39 pace)...and 8 miles would be 44:56 (5:37 pace). So, it seems that maybe 12k would be the equivalent effort. So, in this case, 7.5 miles at MP pace would equal a 4-mile tempo run.
I could keep putting up numbers, but most people aren't really interested in the math. The formula would be simple, but I don't have anything here to develop the formula based on data points.