Keir - Again, being disappointed with pace is something to forget about.
If we come back to the basics of training, it is:
Run plenty miles and run a few of them a bit faster.
Thats it in a nutshell. During this period the only purpose of faster stuff is to keep us in touch with speed so that we dont go suddenly accelerating like mad in a few weeks time and start snapping things.
When I do a parkrun I just go for a good graft but certainly dont strain anything. It is more important to be concentrating on running efficiency and keeping everything nice and tidy at speeds a bit quicker than you're going to be dealing with on marathon day.
Just go through a routine of 2-3 mile w/u and then run 3 miles of effort, whether that turns out to be 6:10, 6:19, 6:30 will not affect your marathon time. You will be running much quicker than this in a few weeks but it will be due to the overall training load, not due to the speed you run these 5Ks at.
Again, it is not a case of faster is better. So to answer your question I wouldnt change away from doing them in the morning if the only reason was that you think you might be able to run them a few secs quicker later in the day.
Going back to yesterday's discussion it is interesting that running is just about the best stress relief activity there is and yet a lot of runners manage to turn it into a stressful activity. The body can actually train itself - if you programme into the brain that you want to run a marathon on April 21st and just go out for a run 6/7 days a week there will be some days where the legs naturally want to go long, some where they naturally want to go fast and some where they want to do neither.
But the chances are that just by stepping out of the door 6/7 days a week and going with the flow you will end up in pretty decent shape on marathon day.
All this structure that gets handed down in schedules is mainly because it takes some time to become au fait with, and trusting of, the body's natural training inclinations. So the structure should be a means of assistance in getting from A to B but if structure itself starts producing stress then surely it has become self defeating?