I was never true elite, but I posted some solid times (1500 under 3:45 and 5000 under 14:00) and competed seriously for roughly 12 years total. At my peak I was running 110-120 miles per week, including two workouts and a long run every week. I would go months without ever taking a day fully off, my one "off" day of each week consisted of a relaxed 60-70 minute run. Most days I would run 15-20 miles total, and on workout days much of that would be at quite fast paces. For example, one of my staple sessions; 40-45 minutes in the AM, workout in the PM consisting of 30 minute warmup, 20x400@63-64 w/ 1 minute recoveries, 30 minute cooldown. All told that day was usually 19-20 miles with 5 miles at ~4:15 pace. The day before and the next day would both be 40-45 minutes in the AM, 70-80 in the PM. The day before each workout I would also do 8x100m strides, not super fast but still adds something. Then I would have another day of 40-45, 70-80+strides, then 40-45 and another workout (probably something like 8x1 mile@4:50-5:00 w/ 1 minute recoveries), then a 2:00 long run usually averaging around 6:00 pace, sometimes a bit faster. I would do this in back to back to back weeks, then run a rest week of maybe 90 miles, with just one workout, then I would be back up for another three weeks. I did this kind of training for my last two years of competitive racing, and ran all of my PRs during that time from 1500m to the marathon. Was I tired a lot? Heck yeah I was. Those morning runs especially were killers. I would often take the full 45 minutes to hit 6 miles, I would be so tired from the work the night before. However, I made sure to never go into a race tired. I didn't believe in training through races; either I was fit enough to rest up for a race, or I wasn't ready to race. I will also say; I could run a solid workout (like that 20x400) then take two days of normal training and just one day where I ran a single 60 minute run instead of my usual ~2:00, and I could feel totally fresh in a race. You really do get used to the mileage, it's just the workouts that can stay in your legs. One extra lighter day between a workout and a race, and you won't even feel it.