Here is a 12 week running program you can follow to get in great 5 to 10km fitness

If you can only run 3 days a week and you have a running background (3 to 5 years) and have been running regularly and have done intervals in the past and have been running for an extended period of time the past 6 months.  Try the following:

1 threshold tempo run. total running time 40 minutes to 60 minutes. between 10 to 40 minutes in threshold/ tempo zone. 70 to 85% heart rate
1 easy run with an acceleration speed focus run. total running 40 minutes. working up to 8 to 10 x 200metres with a 3 to 4 minute rest.  65% heart rate (except on the accelerations)
1 long run. total running 60 to 90 minutes unless you are training for a half marathon or marathon then you need to go longer. 65 to 75% heart rate

If you have that 4th day and you are serious add Intervals.  V02max would be your 3 to 5km pace which might be between 88 to 92% of max heart rate.

Remember this very important summary from an exercise physiologist and elite coach.
Greg McMillan@GregMcMillan

Your cardiovascular fitness can improve at a faster rate than your muscles, tendons, bones, ligaments & fascia can tolerate. This is the reason so many runners get injured. Your plan must respect this & allow time for the musculoskeletal system to “catch up” & become more durable

TEMPO/THRESHOLD

adult threshold/tempo schedule (warm up for 5 to 10 minutes with 65 to 70% heart rate then move up to
wk1 10 minutes wk2 13 minutes

wk3 16 minutes wk4 16 minutes

wk5 19 minutes wk6 22 minutes

wk7 26 minutes wk8 26 minutes

wk9 30 minutes wk10 33 minutes

wk11 36 minutes wk12 39 minutes

SPEED FOCUS (once a week during after a 8 to 10 minute warm up start with drills then accelerations, running these at 80 to 85% of max, gradually building up and holding 85% effort in the middle half of the distance then slowing back down) walk back to the start)

wk1 3 x 80 meters  wk2 3 x 100 meters

wk3 4 x 80 meters  wk4 4 x 100 meters

wk5 5 x 80 meters  wk6 5 x 100 meters

wk7 6 x 80 meters  wk8 6 x 100 meters

wk9 7 x 80 meters  wk10 7 x 100 meters

wk11 9 x 75 meters wk12 10 x 100 meters

LONG RUN (start with the longest run you have done at least 2 of the past 8 weeks with the most recent within 3 weeks).  you have to judge this as it really depends.   very slow addition of time.

wk1  60 minutes wk2 65 minutes

wk3 65 minutes wk4 60 minutes

wk5 65 minutes wk6 70 minutes

wk7 70 minutes wk8 65 minutes

wk9 70 minutes wk10 75 minutes

wk11 75 minutes wk12 40 minutes

INTERVALS

week 1. 8 minutes (v02max). 4 x 2 minutes with 2 minutes rest.
week 2. 12 minutes. 3 x 4 minutes with 3 minutes rest OR 3 X 1KM TEST SET.
week 3. 15 minutes. 5 x 3 minutes with 3 minutes rest
week 4. time trial 3km.

week 5. 18 minutes. 3 x 6 minutes with 3 minutes rest
week 6. 21 minutes. 7 x 3 minutes with 3 minutes rest.
week 7. 24 minutes 6 x 4 minutes with 3 minutes rest. OR 5 TO 6 X 1KM TEST SET.
week 8. time trial 3km or 5km.

week 9. 25 minutes. 5 x 5 minutes with 3 minutes rest
week 10. 27 minutes. 9 x 3 minutes with 3 minutes rest.
week 11. 28 minutes. 7 x 4 minutes with 3 minutes rest. OR 7 X 1KM TEST SET..
week 12. race 5km or 10km.