I run quite a bit. At the moment i do it by distance. I'm rusty at the moment as i have just come back to it literally this past week. I ran a circuit of about 4km everyday this week - (sometimes morning and evening), but my fitness is shocking atm. I will contine to run 4km (but i will do it twice a day this week) and next week, i will extend the circuit to 5km. My fitness used to be quite good.
It greatly depends on how your fitness changes. For instance, it only took me 3 runs this week to notice a real change in time over the same distance. Best thing is to take it slowly. If you want to run a lot, sometimes little and often is better that hammering yourself for an hour.
Try working out a circuit (footpaths - roads etc.) and then do it early morning and late evening. In your case, do 30 mins morning, 30 mins evening (if you feel you can handle twice a day). Then increase both running times by 5 minutes or whatever. Personally, i feel doing 4km (2.5 miles) twice a day is enough to really get my fitness progressing. Drop me a PM at the end of the week, let me know how you feel your runs are changing.