First off, you will be on the same server AND map for 4 hours (Half Marathon) or 8 hours (Marathon). This meaning, you don't want to get bored. I recommend you do this with a friend to spice things up a little. You will want to download 'Log Me In Himachi' which allows players to connect to eachother on a private server. Make sure you load up Sandbox and have lots of mods. See my Gmod Collection for over 20GB worth of mods to choose from.
When you're ready, go to 'Start New Game' and then put everything on the right hand corner to '9999' so you have unlimited fun. Then you would like to slect a big map that you have all the textures for. I use Free Space Extended which is a large map that has everything you needs. Nuclear Testing Facilities, Different Sized Ramps, Water and Docks, Houses and other Buildings. It has everything you would want. Then when you're ready, move on to the next step.

To create a server, you must have Log Me In Himachi. Once you have it, go to Network (on Log Me In Himachi), Create Network and then type in a name and password. Try something like, Thor's Gmod Server and for your password, do you're friends real name or something like that. Once you done this, you will now have your Network up and your friend(s) will have to visit the Joining Section of this guide. But for the host, is all you do once you have it up is load up your server and then it's your friend's turn.

For those of you who want to join your friend, you will just simply have to open up Log Me In Himachi. Once it's loaded up, select 'Network', 'Join Existing Network' and ask your host to give you the Network ID and Password. When you've got into their network, you will then have to RIGHT CLICK on their name in that network and select 'Copy IPv4'. Then load up Garry's Mod, open the console using ` or whatever key you assigned it to (check you key bindings in the options menu) and then type into the console, 'connect' and then leave a space in between a on your keyboard press Ctrl + v.
This will paste the IPv4 into the console, then press enter. If you did it wrong, try it again. If you did it right, it should load up the game. OH and don't forget that you need to have the same mods as the host to play on his/her server.

Ok. Now that you have all your friends in one private server, you need to tell them the rules. Here are the rules:

Don't use too many Nukes at once.
Don't spawn more than two Dupes inside of eachother.
Don't spawn in too many Dupes.
Avoid loud noises.
Avoid being in the blast radius of a nuke.
Don't try to be Kim Jong Un.
Don't spawn in things that have error signs.
Remember to use Utilities, Clean Up, Everything.
Try not to use too many thrusters at once.

These are the rules that will prevent your game from crashing during gaining the Half Marathon or Marathon Achievement. Trust me, it's gutting when that sh*t happens. But before you leave this guide, don't forget, CLEAN UP EVERYTHING IS YOUR FRIEND!!!