Even if OP was in peak physical fitness his thoughts wouldn't just go away, in fact they would likely eat at him the entirety of his workout. Thoughts are what feed existential depression, the best cure for existential depression is to stop thinking about that shit and start living in the moment, which is what most of the drugs try t help you do..

"Yalom (1980), who is perhaps the most widely read current Western writer on existential psychotherapy, describes four primary issues of existence (or "ultimate concerns")—death, freedom, isolation, and meaninglessness. Death is an inevitable occurrence. Freedom, in an existential sense, refers to the absence of external structure—that is, humans do not enter a world that is inherently structured. We must give the world a structure, which we ourselves create. Thus, we have social customs and traditions, education, religion, governments, laws, etc. Isolation recognizes that no matter how close we become to another person, we will never completely know that person and no one can fundamentally come to know us; a gap always remains, and we are therefore still alone. The fourth primary issue, meaninglessness, stems from the first three. If we must die, if in our freedom we have to arbitrarily construct our own world, and if each of us is ultimately alone, then what absolute meaning does life have?"

source: http://www.davidsongifted.org/db/Articles_id_10554.aspx

These questions won't just go away if OP starts exercising to feel better. His mood may elevate and it might make him regress to feeling how he was a few weeks back but there's a limit to how effective this can be and there have been plenty people of peak condition who still want to die over it. Existential depression is kind of a thought disease, brought on by thinking about that crap, so you have to take those thoughts, get rid of them, and replace them with thoughts that aren't so derisive. The main awy they help kids who have this problem is to distract them from it, long enough to get them stop thinking about it since they often won't regress back into thinking about it again and can live contentedly.