Well, i'm playing an extremely relaxed way, and managed to hit 70 in 8 hours more or less and i'm now at nearly 50 paragon, running T1.
The "problem" is not Adventure mode immediatelyt unlocked, but the fact people aiming to "first" will always prepare and look for the fastest and optimal way to accomplish things. If the point of season was to be the fastest one to reach 5k achievement points, there would have been someone with a video explaining a route to do so in the less time conceivable.
No matter the way, season are like that - lock or no lock, expect to see things done in a matter of hours by some people that then will just leave the game or return farming on their mains.
Fortunately, rewards from seasons are not tied to conquests, and you can get everything (xmog, banners and such) playing normally and i must say pretty easily.