I haven't managed to finish a 20 km challenge in a week, but I've been averaging about 10 km per week. Here is what I've done this week so far:
Monday - 3.2 km - 24 mins (treadmill, incline of 3.0)
Tuesday - 3.2 km - 22 mins (treadmill, incline of 3.0)
Wednesday - 5 km - 35 mins (http://runkeeper.com/user/354193968/activity/74148060) P.S., I hit stop on runkeeper while I was checking my distance, I kept running for another couple of minutes to make up the 0.1 km. And I had to stop a couple of times.
After I run I usually do a fitness class or just do weights on my own. On Monday I did a 45 minute abs and legs class, Tuesday I did an hour long step aerobics/weights class, and today I'm planning on doing an hour long weights class and a 30 minute tabata class.
That's basically what my workouts consist of. I try to do this at least 4 times a week. Once it gets warmer out (like today) I'll also be running outside instead of on the treadmill. Today is beautiful out, I actually had to take off my jacket while running and I was still warm with just shorts and a tank top on. Yay warm weather!