First, if you check the calculators FAQ, you will notice the following:
These calculators should be considered guidelines. Obviously, some (such as the distance conversion calculator) are based on hard numbers. The others are based on research but can not be guaranteed to be perfectly accurate. If you ran 5k in 18:00, that doesn’t mean you will run 10k in exactly 37:31. However, it would be reasonable to assume you would be fairly close to that time. Also, if you ran that 18:00 5k, your ideal easy run pace may not be exactly 7:29/mile. However, that is most likely a good starting point, from which to find your ideal pace. Use these calculators to get an idea of what you may be capable of. Don’t think they are exact because no calculator can be exact. After all, we are people, not machines.
Now, confession time. I set up the training paces calculator to give suggested training paces based on the work of a certain Mr. Jack Daniels. Personally, I have always been a little hesitant about the easy run paces Daniels suggests and I have always considered them to be “high end” paces. In other words, I consider that to be a pace that, at least most of the time, I should not be going faster than but I can go slower than.
I believe part of this is due to my training volume, as might be the case with you. If I started running 8 miles a day right now, running at my suggested easy pace (currently about 20 seconds per mile faster than I normally do easy runs) would be a lot more reasonable and maybe even conservative. However, at 15-20 miles a day, it becomes an occasional thing at best.
As I note in what I quoted above, consider those suggestions a starting point. It’s impossible for a computer to calculate precisely the right numbers for all of us because there are simply too many variables. My aim with that calculator is to give you a tool that you can use as a starting point to estimate where you should be. Once you are out on the road, adjust as necessary. My bet is that the most frequently needed adjustment to those suggested paces would be a slower easy pace.