What is this all about?
If you are talking about the speedruns, the game is basically build up with campaigns which consist multiple scenarios. It is not really one continious game in single player.
The game is mainly run at the moment by beating each individual scenario as fast as possible. Many levels are quite tricky and we fail more often than that we get the strat perfectly done, partially because of RNG
Maybe more full campaign/game runs later, but first every level has to be routed and needs a strategy that can be done consistently.
If you want to start running, either pick a level you like and go for it, or watch already recorded runs and find their weaknesses and improve them.
Yeah as SiFly mentioned, the most common Age of Empires I / II speedruns are individual level speedruns. There are a few full-campaign runs in existence but they are not that optimized or anything.