Endurance increases the Energy Cap while Efficiency makes you run using less energy.
The main idea for Endurance is to have a property increase when an upgrade is bought and check what property amount (endurance lvl) the player has to make them not ovveride each other.
(Ex. player buys upgrade that increases energy cap from 5,000 → 10,000 and buys an upgrade that increase energy cap from 1,000 → 5,000)
For the efficiency upgrade, it’s more or less the same but with movement meters.