My body recovered suprisingly quickly from the caffeinating and the lack of sleep.
Anyhow, I've decided to start carrying at least 7.5kg with me everywhere, to "trick" my body into thinking I'm heavier and putting on the muscle to support that extra weight.

First day did it in a satchel. Walked about 2hours, overall. Altered positions (holding in one arm, using only fingers, bent elbow, both hands, behind back "carrying a menhir"-style) to exercise all "lifting" muscles at least a bit. Arms tired, but only a bit sore.