Everyone and his/her cause has a walk for something. My husband died of cancer, and the people who helped us most weren't walking around, chatting with their friends and eating junk food under the spotlights through the night (Relay for Life) or stopping traffic in the roads.

Helpful people took care of our children when Daddy was in isolation and the doctors told me I needed to stay with him; they drove the kids to the hospital when they could visit; they made food and cleaned our house. One of them took my little guy to her house every Friday morning, fed him lunch and took him to afternoon kindergarten. They stayed with my husband so he wouldn't be alone when I went back to work. They sent cards–and money to pay our bills.

People will give whatever they will. They don't need you to walk a certain distance or amount of time. If you want pledges, ask them to pledge for every can of nutritional supplement you take to your local oncology office for patients who can't eat much, need to keep up their strength and can't afford to buy the supplements. They can pledge for every patient you drive to the office or hospital for treatment, every yard you mow because the homeowner just doesn't have the strength now, every patient you take to the grocery store, church or concert; every one you go to visit.

The reason to do such a thing as you describe is to build fellowship among the people who do it. If that's your goal, okay, but It's not an efficient way to raise money.
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Went down that road, wouldn't have made it without help.