Well, I ran at the pace at which I can run longer. So I ran 10k at about 5:10/km and then another 10k at about 5:30/km because I was a bit tired and the last few at about 5:45. So I was getting tired while running, but I didn't feel exhausted afterwards. I mean I could have tried to run faster, because there was an ambulance parked in the park, but I wanted to have a fun run. :))
I think Will is asserting that fun = tired. Also, I'm not sure that feeling tired is the only indication (or even necessarily a good indication) of a productive workout. There are all sorts of benefits to "long slow distance" (say 1-3 mins/mile slower than 10k race pace), including improved aerobic performance, muscle training, heat regulation, and recovery. You simply can't sustain 100 km/week of speedwork. I think most training plans have 50%+ of their mileage as slow-ish.
I wouldn't say that fun = tired, more like slow = boring. I applaud your discipline, because my problem with runs that are supposed to be slower or longer or whatever is that I inevitably get bored and start going faster than I should. But on the positive side, you finish quicker and have more time to eat raspberry jam.
"have more time to eat raspberry jam." <-- Good point. Run faster!