nmg85 don't beat yourself up about it. You will have good and bad days and if you're still relatively new to running it will take a while to build up the stamina. If you do have to walk, at least make sure you cover the distance as a whole, at least what's what I've been doing. I also found that a bout of walking did not add much time to the total, as it was either walk and then run better again or keep going at a much slower pace, end result virtually the same...
Lots of factors: how cold it is, when did you last eat, what did you last eat, are you hydrated, are you rested or did you strain yourself too much on Monday, are you in the right frame of mind. Is it a different route? Grass (I find that much harder) or pavements etc etc.
WHY did you think you had to walk? I ask because I've been there for weeks, much much walking. In my case my calves and ankles hurt, really hurt. I was not tired nor out of breath... On Monday for the first time I reached 10 minutes and they did not hurt, so I kept going. By 30 mins I was tired and glad to stop when I reached 5k. As soon as I stop to walk it exacerbates the problem and I can't keep going for any length of time at all. I think there is some technical explanation to do with build up of lactic acid or something, but I also know that for me it's all in the mind.
Your next run will be another day, and for the fun run at the end I am SURE you will make it, you've proven it. As long as you keep practising up to then you will not lose anything you've gained up to now.