A trick for working with epoxy is that it cures much (much!) faster in a container than when spread out on a surface.

This is because when epoxy cures it gives off heat.
In a container the heat causes the mixture to react even faster, giving off more heat...
You get the idea...

Once the epoxy is spread out, its temperature is held essentially constant by the higher mass of the substrate.

Whenever I'm working on a large joint or surface, the first thing I do is pour out the epoxy from the container across the workpiece.
From that point on you have a lot more time to spread it out to the desired thickness since it's not curing as quickly as it would in the pot.

Another advantage of high-performance epoxies is that many of them have a number of different hardeners that will change the cure-time (and also usually the viscosity).
For example, WEST's 105 System has three hardneners: fast, slow, and super-slow "tropical".