There's this voice in the back of your head saying... maybe these are wasteful miles. I feel like shit, I'm not running fast, and I can't remember what I did between the last 5 times I've come here to do this unquantifiable thing. This is a curse of the productivity oriented culture. Think of it like this... what would you be doing instead? Accomplishing something more important than being active? The mind tries to sabotage your training by planting doubts like - maybe your running too much! No such thing. It's cumulative. If you're running 100+ miles a week and all of those miles are getting slower and you feel more and more tired each time, then you have room to cut out some junk miles. In order that a day or two each week you feel strong enough to run fast. Otherwise getting that heart rate up to improve fitness and teaching your neuromuscular system to run by memory/repetition will greatly benefit you in the long term. The single most important factor in improving your speed/strength/endurance is mileage. Ideally you want to achieve high mileage by running a moderate pace every other day and make sure that you don't feel tired when you start your stronger runs on the other days. As long as you have enough energy to run with good form and keep the same pace throughout(negative splits are even better), then mileage is only going to help. I sometimes feel tired starting runs but I go slow and by the end I feel good to hit race pace for at least a mile or two. That lets me know that I'm not overtraining. It helps to run outside b/c I can always say that at least I'm enjoying being outside. But on the treadmill or on the trail you've run countless times just remember improvement is cumulative and every time you train you compact the effect of all runs before that. One of the greatest things that an endurance runner has to overcome is the boredom and doubts and loneliness that comes with the numerous miles and hours you will spend doing an unremarkable, sometimes painful, and tedious activity.