Try not to worry ! These types of charity walks are for fun ! take your time and just enjoy the day.
One thing I will ask you is what sort of shoes are you wearing ? Were you fitted for a decent pair of walking shoes ? Many aches/pains can be traced back to incorrect footwear. When a person walks or runs, their feet should have good support. So, if you're wearing old shoes, that might be one possible cause.
Also, you don't want to wear brand new shoes if you're not used to walking longer distances. try to wear shoes you've worn for at least a week so that you know they won't cause blisters. Although, for 5K, you shouldn't have to worry about blisters, but you never know.
Actually, you'd be surprized how much energy you'll soak up from all the other walkers around you. I've done a bunch of 5k and 10K runs as well as charity walks. Everyone is very supportive !
Enjoy yourself ! Don't worry and don't fret ! Just have fun !