I ask because I seem to really struggle at medium hard pacing and I don't understand why. I can complete interval and repetition workouts that would imply I could run an 18 min 5k (5:45 mi). However, I keep ending up running a 5k (not in a race) around 19 min. I only bring this up to give you an idea of speed fitness. But, I'm currently wondering about my overall endurance or where my lactate threshold is. For example, I ran a 19:04 5k by myself about a month ago. Around the same time, I was really struggling to run 5 or 6 miles at 7:15. But, I'll run long runs of 8 miles, sometimes longer on around 30-35 miles a week. I realize more miles would help here, but with work and my tendency to get injured, I'm trying to avoid going to deep in more miles. Today, I ran 6 at 6:52 and felt pretty good - could have gone further or maybe slightly faster. The reason I'm here is this, every kind of pacing calculator says I should run tempos closer to 6:20 or 6:28, or here's an article on lactate threshold that says I should be able to maintain a pace of about 15-20 seconds slower than my 5k time for an hour. While I'd love to run this pace without a care in the world, it's just pretty damn hard. Is it really just more miles? Is it overall toughness? What am I missing?