For 5k I'd run 4 x 1200m at current pace for 3 consecutive w/o's, one session a week, then if not having raced during that period I'd try speeding them up a bit. 2/3 seconds a lap. If I had raced during that period I'd adjust my pace accordingly. Usually that was do-able, depending upon weather.
Actually high milage guys have a better aerobic system than your average 5k guy. Men that could beat me in a 5k, I could run 90% of them down the last mile or so in a 10k. I always told my track distance runner at the end of the season, "Summer miles bring Fall smiles."
If you want to be a better runner then you better get out the door at least twice a day. If you are a high schooler or college runner with no obligations over the summer you could probably run three times a day.
If you don't know the correct way to double or triple, start a thread and get advice on how to do it with wearing yourself down. There are many, many experienced and knowledgable posters here that would be more than happy to guide you thru the process.