[Fallout]

Donating is the fastest way but not so great if you are on a budget.

Using AL cash to buy credits off the market is your next best option but can be a slow process due to the fact that large amounts of credits are not always readily available.

Athletics offers the best career choice in increasing endurance as it has the highest conversion rate but it isn't that spectacular so it is also a slow process.

Pick what suits your situation best.