This was pretty much my go-to, hardening summertime workout in high school. No time, because there's a party to attend? Head out the door at 5K PR pace and hold it as long as possible. Done.
Warming up is overrated, anyway. You ever see a jaguar warm up before running down prey? Didn't think so.
I wouldn't do this now, three or four decades later, but it worked for me back when running a fast 5K was my thing.
(And FWIW, I'd run a high-16 when I was turning high-15s later in the season, or 30 seconds or so from the previous season PR.)
In college, on the days I'd need to get in a second run, I'd do much the same when up for it. Or, a 3 or 4 mile effort out the door, then head back for a 6 or 7 mile outing. Aim for 5 minute pace on the fast ones, then probably mid-6 back.)