steroids will not help much in a 5 k. unfortunately you have to stop lifting heavy for a while until you run your 5 k, that is you have to run 6-8 k every 2 days for 3 weeks and lift light weights twice a week. then after those 3 weeks try to time yourself on the 5K . ( of course doing that you will lose some muscle mass and weight , you probably need to lose 5 lbs total). then back again for the next 3 weeks run 10K every 2 days and lift light weight twice a week. then try a 5 k. Then for the last 3 weeks or so run 3 times a week , a couple of 10 Ks and a 5 K ....for the last week before your run just go for a couple of 5 ks , do not run 3 days before the race.
ephedrine combined with aspirin will help you on the race day, but be careful about ephedrine just a very small dose and only on race day.