"The question is what is your current 5k and/or mile pace?
If you are still in 5:0X shape, this shouldn't take too long.
If you are in more like 18:30 shape for 5k, it could take a year."
IMO this is important - you can do all the dedicated speedwork you want, but if you are out of shape, it won't matter, you won't be able to hold the speed, especially if you are a distance guy.
i came of an injury a couple years ago and spent 3-4 months during the spring/summer doing nothing but sprint workouts and lifting weights. only mileage was warm-ups and cool downs, maybe 10-15 mpw. despite being my strongest (squat and DL PRs in the weightroom), I could barely break 60 for a quarter, would fall apart the 2nd 200m. in the fall after 3 months of more distance-oriented training and getting into 15:30 5K shape on 60-70mpw, I stepped onto the track and ran a 54 (and 1:56 for 800 in the spring).
did all the speedwork and weights help my speed? yes. but I couldn't reap the benefits until I got into better aerobic shape.
(PS - this was at age 29. my HS 800 PR was 2:02. you don't lose speed, you lose what you don't train. until you become a REAL old man that is...)