I'd personally try to do one threshold session and one repetition session per week, the threshold for 5 k could be something like 10 minute warm up, 10 minutes threshold, 5 minutes easy, 10 minutes threshold and 5 minutes cool down, the interval, I'd go something like 10 minute warm up, 6x400m interval with 100m recovery between each one and then a 5-10 minute cool down, over the course of three weeks add 2 minutes per threshold set per week and increase the interval number by 2 per week, you should see some pretty decent improvement even in 3 weeks...
@Dammit put your most recent race pace into here http://www.attackpoint.org/trainingpaces.jsp and then follow a 5k training plan using the paces it suggests.
I'd personally try to do one threshold session and one repetition session per week, the threshold for 5 k could be something like 10 minute warm up, 10 minutes threshold, 5 minutes easy, 10 minutes threshold and 5 minutes cool down, the interval, I'd go something like 10 minute warm up, 6x400m interval with 100m recovery between each one and then a 5-10 minute cool down, over the course of three weeks add 2 minutes per threshold set per week and increase the interval number by 2 per week, you should see some pretty decent improvement even in 3 weeks...