Ok, I will guess at what you are talking about. You want to participate in a running or walking event where people pledge to donate money based on the length of your running or walking.
120 km is about 75 miles. A very tough warrior can run that far in a day. To go that far in 31 days would be 2.4 miles or 4 km per day. Any fit young person can walk that far, or run with a bit of conditioning.
Get a ruler in your hands. Measure things until you start to understand how a ruler works. Measure some stuff and figure out where the center is. Say you measure a book and it's 7/8" thick. You look at your ruler and see that every eighth is divided into two sixteenths, so obviously half of 7/8" is going to be 7/16". If you write that out you have 1/2 x 7/8 = 7/16. And you notice that 1/2 is divided into 2/4 and then into 4/8 and so on, so you can convert anything to anything by multiplying all the numbers on top and then all the numbers on bottom.
Other rulers are divided into 10 and 100 parts. But an inch is still an inch, so anything on one ruler can be translated to the other ruler. A half inch on one ruler is 5/10 or 50/100 on the other. An eighth inch is just 12.5 marks when you have 100 marks per inch. A metric ruler divides an inch into 25.4 parts, so a half inch would be 12.7 of those parts. Pretty simple, isn't it? Practice this a bit and people will think you went to wizard school.
That definitely doesn't sound very much... thinking on that, I think I could do around 10km a day, so will definitely up my total :) Thanks for your help! Sorry I must sound stupid, just needed some clear head space, thank you!