Getting to the Expo
The Marathon Expo is located in a large convention/sports center known as the "Parc des Expositions" and is some distance from the marathon course. The Paris Metro is probably the quickest and lowest cost method of getting there. The closest station on the Metro is Porte de Versailles on Line 12 (see Metro discussion here). The station is only a few steps from the "Parc des Expositions" complex entrance. The specific building that the expo is located in varies from year to year, but there is always plenty of signege to direct you. Line 12 connects with Line 1 at "Concorde" (see Friends and Family discussion related to the Metro and Line 1 here). Or, it connects with 4, 6 and 13 at Mountparnesse. Line 6 loops from "Nation" (see Line 6 and "Nation" related info here) down around the southern part of the city and then ends back up at Charles De Gaulle Étoile. That gives you lots of options.
If you are coming straight from CDG Airport, take the RER to Gare du Nord (the first stop after the train goes underground) and transfer to Line 4. Then ride Line 4 all the way down to Mountparnesse. Then transfer to Line 12 as mentioned above. From Orly Airport, there is a short transfer to the RER where you head north to either Line 2 or 4 at Denfert-Rochereau. Both lines will take you to Mountparnesse where, again, you can transfer to Line 12.
That probably sounds complicated. If you have a lot of baggage, it will be and you should probably go to your hotel first and lighten your load. If you are traveling light, you won't have much trouble with it. Get a Metro/RER map and study the routes before you venture out. It will cut the hassle down to a minimum.
The organizers list bus lines 39, 42 and 49 all of which go to the general area of the expo. I do not have direct experience with them. If you are on a budget, I would not recommend a cab unless you are within a short distance of the "Parc." You could buy a lot of stuff to take home for what it would cost to "cab it." Then you have something to show for your effort and that works a lot better than describing your great cab ride to your friends. Renting a car and driving is a good option, but only if this is your 8th or 9th trip to Paris and you have done the "car thing" before. (rs)