These are frequently called "threshold runs" by some, but the more I've studied anaerobic threshold, ventilatory threshold, lactate turnpoint, etc., the more I've become convinced that it's all meaningless, but still a useful fiction. These are not supposed to be hard runs (nor easy runs, for that matter), but they are the most difficult for me and I'll discuss how I've dealt with them.
First of all, a fast continuous run is simply one-third of a race and is between 15 and 75 minutes in duration. The two standard versions of this are: 20 minutes at a pace you could race for one hour and 60 minutes at a pace you could run for three hours (conveniently marathon pace for 3 hour marathoners). The runs should be part of a medium-long run of 15-20% of your week's mileage. One week you might run a 1 or 2 mile warm-up, then 20 minutes fast, then finish the duration of the run, which tends to be difficult because one's tired from the faster section. Alternate weeks, you run the majority (at least half) of the time you'll run easy, followed by an hour done harder; the idea here is to get a feel for running a hard pace when already tired. The hard parts of these two runs should total no more than 10% of the miles you run in the two weeks.
Back when men had to run 2:50 to qualify for the Boston Marathon, the standard was to run 70 miles per week. One week, one would run 10 miles at marathon pace. The following week, one would run 4 miles at 1/2-marathon pace (though, at the time, the 1/2-marathon was a rare event). These 14 fast miles are exactly 10% of the 140 total run in the two weeks. One still sees remnants of this pattern in modern training schedules.
I personally find these to be the hardest runs I do. I'll do the easy part too hard, making the one hour that was supposed to be faster impossible. Or I'll start the hour much too fast and fall apart. Or I'll push for the entire hour, but be completely exhausted and unable to do anything for several days because I ran too fast.
The problem is in pacing. I find the difference between 5K and 10K pace to be huge, the difference between 10K and 1/2-marathon pace to still be large, but the difference between 1/2-marathon and marathon pace seems razor thin. I've tried running with a GPS watch that warns me if I go too fast. I've tried running by heart rate. The problem really seems to be that I don't know for certain what "pace I could race for one hour" actually is, that I might not know what shape I'm in, that the weather influences pace, that I might not have slept well, etc. Having a sense of how one feels 1/3rd the way through a race is key.
The only remedy I've found is a bit extreme, but it works for me. I convince myself that I'm actually going to do a full race in the workout - let's say a marathon. I wear what I'd wear in a race, do all my pre-race preparations, then plan to race an entire marathon. I just stop when I get to 9 miles or so.
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