Let me say, right from the outset, that I am not recommending people not to run for charity. I have done it several times, but what I would advise is not to train for a PB and run for charity, pick a race that you can just enjoy, perhaps use it as a stepping stone for a later race, attempt a PB then.
My reasons for saying this comes from bitter experience and I also know that one of our members is going through the same thing that I did.
The main problem with running for charity is that, as soon as you have signed up for it, you will feel honour bound to do the race, regardless of your condition, injured or not.
I experienced exactly this the last time I ran The London Marathon. I had agreed to run for the local Playgroup (which my wife Debbie used to run) as they needed money to build fencing around the building and garden they used for the kids safety. So I trained hard for 3 months and all was going well until about 7 weeks before the race I started to get pains in my knee. I decided that I could train through it, but, as you would expect it just got worse.
Now, if I hadn't been doing it for the Playgroup I would have pulled out of the race and rested up for another race, but, yep, you guessed it, I felt duty bound to run the race, I didn't want to let the kids down. So I didn't train for 6 weeks before the race, hoping my knee would sort itself out.
I went down to London and felt ok, my knee felt good, so I started off at 7 minute mile pace, which would get me round in just over 3 hours. I got to the halfway marker just passed Tower Bridge and started getting pains in my knee and by 15 miles I was in agony, so I started to walk. Needless to say I did finish the race in a personal worst of over 5 hours and also ended up not being able to run for 3 months afterwards, but at least I got the money for the kids. Ironically I found out that the injury was in my calf, the pain came out in my knee!
The moral of this story is, by all means run for charity, but remember you are committing yourself and your body to it.
I hope this doesn't put anybody off running for charity, cos that's not the idea, just beware of the dangers.