My father in law has MS and they just got a walk a thon going in our area 2 years ago. My 4 boy's do the walk every year now and they don't really set an amount that they need to raise. You can set a goal and they have prizes for amounts a person collects, like a t shirt if you get $100 in donations and such, I can't remember the prises you can get, just the t shirts because that is what my boy's got
We did the March of Dimes walk also but again we did not have to collect a ton of donations for it.
I am not sure how other's work their walk a thons so it could be a lot different depending on what place you will be walking for. Usually any amount a person can get raised for the event is much appreciated.