I often do two runs a day, more through circumstance & convenience but it seems to be paying off as my times have improved significantly.

For the last 8 months, 3-4 times a week, I run 3 miles at 5k pace with a sack on my back (probably 6:45 but I don't have a watch), then nip on the train for 25 minutes & run at same pace for 1 mile to work in the morning. I then do the reverse in the evening, at a slower pace as I tend to be tired from a day at work.

Probably against the grain of recommendations but, again, commanded by circumstance.