I know I've said this before, and I know I'm a bit of a lone voice in putting forward this view, but sometimes when our body tells us to stop, we should listen!
I think the ability to recognise when we would be better to go slower, or shorter, rather than faster or longer, is something we develop as we become more experienced runners.
Case in point. On Monday I set off to run a slow 10 km, but gave up and walked home after 8 km. Why? Because my calves were aching and I was too hot. The over-heating was due to starting at 10.30am, but I needed my sleep after only 4-hours kip the previous night. The aching calves were from dancing at a music festival for several hours on Saturday night.
I suppose I could have pushed myself harder, but I think I know my body (and my 'runner's body') after one years of running, to know when to listen. I'm currently building up my distance again following the second injury in 6 months, so feel I'm wise to ca canny with my knees and leg muscles. Plus I'm currently cycling a total of 96 miles per week for my commute (30 miles round trip some days, 15 others) and this leaves me physically tired.
I read somewhere that we should 'Sleep one extra minute per night for each mile per week that you train' and am wondering how this translates in terms of miles cycled.
Today I tried again to run 10km and again after 8 km I felt like stopping, But today my thoughts of "My calf is aching" was more just a general moan to myself - not a genuine injury! So I gave myself a good talking to and as I emerged from the woods onto the road, not only did I make myself complete the the remaining 2km, but I increased my pace. 10km done in 64 mins. Not fast; but I always perform better in cold, not heat and am not even going to try running fast in the current warm weather.