running 5 times a week

Short answer: Yes.

Longer, more boring answer:

Disclaimer: My salad days as an amateur distance runner was in the late 90s, so any knowledge I have is from stuff like THE RUNNERS HANDBOOK, RUNNING LORE, and magazines like RUNNER'S WORLD and RUNNING TIMES -- from 8 years ago. Plus, my own meager personal experience. (Anecdotal evidence = bad sampling.)

But to my knowledge, with beginning and intermediate running training, the running volume can be essentially self-paced, and this includes days running. In other words, if you can handle 5 days, run 5 days. If you can handle 6 days, run 6 days. Some hardcore running enthusiasts will sometimes take no days off at all.

Sidebar: It should be noted that hardcore runners, like anything hardcore, can overdo it. For example, there's a subculture of runners who only run barefoot. It apparently works for them, but the point is, like with daily running, it might not be for everybody.

In any case, conventional wisdom was that running hobbyists should vary workouts among lactic acid threshhold training, VO2 max training, and LSD (long slow distance) training, alternating between heavy and light effort from day to day.

But since you're just starting, everything probably LSD training as you build endurance. So if you want to run day to day, I'd suggest trying to alternate between heavy and light effort. Save tuesdays and thursdays for shorter runs that still burn calories, but not necessarily do much to improve your abilities (aka "junk miles"), and keep monday/wednesday/fridays for days when you either push speed or distance.

Whatever you decide to do, hang in there, and when the 5k comes, good luck.

Good advice from Greggers. But if you are able to handle the early weeks in the 9 week program, and have energy in reserve, I would suggest a few things instead of just adding more days running.

1) Skip ahead a couple of weeks in the program. If you can already run you don't need to start at baby steps 101. Then you are doing harder sessions and will benefit from days off as per the plan

2) If you don't find them difficult, run faster. As the runs get longer you might need to slow it up (I know I did when I was doing this plan approx 10 months ago) but if this is all the exercise you are doing, give yourself 3 proper workouts a week rather than 5 easy workouts that don't need days off to recover.

3) If you have the time and energy to work out 5 days a week, why not do the 3 days running as planned and spend the other 2 days doing some other form of exercise. If you are following the couch to 5k plan because you have previously done zero running, and then start hammering the pavement 5 days a week, this may be overtraining and cause you some injury (or just slow your improvements because of a lack of recovery time).

If you use the other 2 days to do some unrelated exercise (sports) or ideally resistance training (weights) you will improve your fitness and drop the pounds. [OK actually doing weights may not drop a lot of pounds as muscle is heavier than fat but its much healthier to have it, improves your metabolism and help you burn more calories throughout the day]. Running is great for burning calories but if you are already doing a few running sessions a week, do something to preserve or build your muscles, as I'd suggest your goal is to drop pounds of fat, not just drop pounds of body weight.

Generally, see if you still have plenty left in the tank for 5 running sessions when doing harder workouts and a good mixed program; if you are just starting out, that sounds like too much running to me