Is sub 20 5k possible for me? (Page 2)

You certainly don't need 40mpw for a sub 20.

Just to echo what has already been said several times. Your weekly mileage is really quite low. 30 mpw is typically a sweet spot for a lot of people. It's where volume and quality meet at a good place while not taking too much out of your life. You can get pretty far on 30mpw, being smart, and consistant year round.

Well my first 5k was a 19:12, given I had been running 25-35mpw for over a year at that point. Slowest 5k is a 20:20 after a busted knee took me out for 3months shortly after that 19:05. ;p I'm ready to be done with this half ironman tomorrow, recover and run a stand alone 5k soon. I always enjoy a 5k. It's quick and it lets you know how fit you are pretty quickly! I've really cut out running races this year, and now I'm itching to do some my self.

How long does it take someone new to the sport--or someone coming back after several years off--to get up to a 30 mpw average, though?  I could see attaining that in my second year back to the sport, but trying to get there in the first year seems like a likely recipe for injury, to me. 

I know that some people do much more than this--some people go couch to marathon in astoundingly short periods of time--but is this wise?  Aspiring triathletes aren't pure runners, we cut our recovery time between runs to train for the other events...