Pammie, I think doing two runs on days when you have loads of time makes a lot of sense. Several coaches advocate several days of harder training (overload) followed by easy days to recover. Perhaps when you have the time you could do one session where you do short fast intervals, with long recoveries, to make the fast bits faster, and enjoy not being in a rush to finihs the session, and then as a second run, something like a very easy relaxed 30 mins. In the weeks when you are pushed for time you could, on some days, just do a hard 30 mins or 2x10-15mins with a few mins easy in between min, so as maximize value-for-time. Both these tyoe of session would be ideal for 5k training