2,000m is a fairly long run, but easily attainable. Just stay alive! Many of the power-up specific levels will have you running this long and longer, so this is very likely to unlock naturally. Powerups such as Gru's Rocket, the Mega Minion, and the Launcher will make this easier to attain, since these powerups will propel you forward for a fair distance without you needing to even worry about obstacles.