There seems to be an assumption here that the difficulty of going somewhere is proportional to the energy required. What about time, and provisions for life su

There seems to be an assumption here that the difficulty of going somewhere is proportional to the energy required. What about time, and provisions for life support during that time? What about launch windows, in terms of the amount of waiting time required before a return trip and the difficulties of a rescue mission? Others have already mentioned that the amount of fuel/weight you need is worse than linearly proportional to the energy required.