Sorry for the delay, hopefully this is in time for boab and cit to plan their pre-xmaslunch runs!
A quick google produced this page which looks at some of the science.
http://runnersconnect.net/coach-corner/optimal-long-run-pace/Here's the summary:
The body of evidence is clear, your optimal long run pace is between 55 and 75 percent of your 5k pace, with the average pace being about 65 percent.
It’s also evident from this research that running faster than 75% of your 5k pace on your long run doesn’t provide a lot of additional physiological benefit. Therefore, pushing the pace beyond 75% of 5k pace only serves to make you more tired and hamper recovery.
In fact, the research indicates that it would be just as advantageous to run slower as it would be to run faster. 50-55 percent of 5k pace is pretty easy, but the research clearly demonstrates that it still provides near optimal physiological benefits.
If you’re feeling tired and the long run isn’t scheduled to be a hard day, don’t be afraid to slow your long run down. My suggestion is to start on the slower side of the pace recommendations (50% of 5k pace) and slowly pick it up through the run if you feel good.
(my bold)
I think you'll agree that 50% of your 5k PB pace is v v slow indeed! Thats ~14min-mile pace for me. I suspect I only jog that slowly between hard intervals, if at all.
Two waffly points that I believe back all this up*:
- at any speed/effort, no matter how slow, your body makes efficiency adaptations. Physios talk about stuff like "muscle firing sequences". This is why a slow run is better than - say - a slow bike ride, for running purposes. And;
- the reasons to avoid overdoing the long run are to keep some energy back for the more focused sessions, and to minimise your injury risk (injuries correlate strongly with your overall weekly training load, especially overtraining).
*(based on papers wot I have read, but cannot quote here!)
