Marathons have a distinct finish line. When you're at mile 20 and your legs are falling off and you take tylenol or whatever they give you to make the pain go away... or when ppl on the sidelines say "almost there!!" and you send them a rude look because those last six miles are the WORST... at least you know you'll cross the finish line at some point and pass out on the grass.
No, startups are like running marathons but in Stephen King's the Long Walk. There's no finish line — you just have to walk farther than those around you, or you die trying.