[handcycle2005]

sw07, those are good 5K times you're running off low mileage. 20:00 is quite realistic.

5K
Once a week, do a long run. Pick a one day a week and add 1 mile every 2-3 weeks until you're up to 8-9 miles. Hold at that point, pick a day halfway between long runs and add 1 mile per week up to 6 miles.
Long run at easy pace, midweek run at steady state pace.

Intervals.

workout 1

2x1200m@current 5K race pace(5:22-5:27) jog 400m recovery in between
Jog 400m
2x800m at goal 5K race pace jog(3:10-3:15) 400m recovery in between
Jog 400m
2x 400m@ faster than goal 5K race pace(1:28-1:30)jog 400m recovery in between

workout 2

3x1000/200 combo(1000m@5K goal pace, jog 200, 200@ faster than 5Kgoal pace) 1000 in 4:00, 200 in :43-:45
Jog 600m in between

If running long Sat or Sun, do these workouts Tu/Thu.

10K
Same long runs as above but you can add repeats to the interval workouts.

1/2 marathon

No intervals, two tempo runs per week 20-25 min. each.
1 long run per week of 10 miles or more. Preferably more if possible up to 15 at an easy pace.

Easy, steady state, tempo paces are given in the calculator I linked to above.

Good luck.