Having physical strength is so you don't have to rely on others to do things, which is not only just for a 'shit hits the fan' situation. The mental strength comes as part of the natural progression of gaining physical strength too. You don't lose the gains the second you stop working out, you'll still have that base strength for a long time to come, and will still retain the majority of the remainder unless you entirely stop doing any sort of physical activity for ages. To be fair though, everyone's interests are different, some find the idea of sitting down doing nothing to be intolerable, some find being up and about intolerable. I think the important thing is what you said: loving yourself means not being UN-fit.