Anyone know what could cause a former runner to not make any progress at all?

I used to run a lot and I would improve FAST. I could run almost 3 miles, full sprint, without stopping, and I would gain an extra 1/10th of a mile maybe every week (meaning if I could run 2.5 miles one week, then I could run 2.6 the next). I never measured it so that’s a guess, but the point is, I never reached a plateau.

Well I stopped running for about six months and suddenly, surprise surprise, I have a short-notice physical fitness test coming up for a job I applied for. So I’ve been running like hell, and somehow, I’m not improving AT ALL. It’s been about two and a half weeks that I’ve been practicing and I haven’t seen ANY change. Actually I’m so bad rn that I’m [b]off the charts[/b] in the fitness model that my job is using.

Help? Lol