Converting marathon fitness to shorter distance fitness

why don't you pick another marathon in your area to run in the next 4-8 weeks depending on your injury?

Lydiard used plyos on hills to build up speed/strength/power and slowly introduce anaerobic running before going to track work, but that was in a much longer time frame than you're probably looking at. Ron Daws just plain ran hills at what he called a strong pace, which I guess we might call tempo effort, but again it was in longer-term development.

Daws said when he went to the track after base work, he had a lot of endurance and not a lot of speed. So he did really long track workouts to begin with and didn't time any repeats. It didn't take very long for him to round into pretty good racing shape doing that. Here's his "introductory interval workout" from The Self-Made Olympian:

2-mile jog,

2x200m, 2x400m, 2x600m, 2x800m; 200m jog after each,

2x1200m; 800m jog after each,

2x800m, 2x600m, 2x400m, 2x200m; 200m jog after each,

2-mile jog

"The first few times through I don't time this workout because (1) I know the times would be discouragingly poor, and (2) if on the watch, I worry about times rather than effort and risk going too fast. To prevent running too fast, I usually allow only 200 meters between each run, and the realization that I'll be covering 14 1/2 miles is also a deterrent. I use this workout to get acquainted with the track, get my rhythm going, and step up the pace a little from what I've been doing on the roads.

After about three of these workouts, I start taking times and rest a 400 between the 600s and 800s as the pace quickens. With four or five of these sessions completed, I move on to others that involve less volume and more demanding speed. Sprint work comes last."

I'm not really interested in doing another marathon, I feel like I put so much work into Chicago that at this point I would rather just take my fitness and focus it on a different goal. Plus I already paid for everything to travel to Chicago and will be taking the trip to cheer on friends. I don't have any good local marathons so if I did another one it would probably cost me another 500 minimum between travel, hotel and entry expense.

Once your body has recovered from the injury pick a day and go to the track and od the following:

run a 2 mile warm-up and do some light form drills and strides, then run 1 x 100 meters all-out followed by a 2 mile cool-down.

Thats it, you'll be ready to set a new 5k PR. Its called adaptation by shock, works everytime.

(also don't forget to drink an IQ shake which help you realize that consistency is the key and bagging 8+ months of work toward a certain distance because of a small injury is pretty silly, just like my advice above).