I imagine a good portion of it is water, glycogen, and food mass in the guts.
In my experience, I can NOT train hard for 2 hours. I can train for 2 hours, and I can train hard, but trying to do both is not a possibility. 2 hours is too long for me to sustain the intensity that drives muscular growth.
If I were in this situation, I’d pick a limited/abbreviated program and see how that works. Dan John’s “Mass Made Simple” is a great choice, as is the classic “Super Squats”. A more modern approach could be Dogg Crapp, as outlined here
Or something like Jim Wendler’s 5/3/1 BBB Beefcake, 5/3/1 Building the Monolith, or the 3 month challenge he outlines here