As always, it depends.. Depends on the end goal. If you can only complete two miles and you're signed up for a marathon a month from now, you're going to have

As always, it depends.. Depends on the end goal. If you can only complete two miles and you're signed up for a marathon a month from now, you're going to have to push past your comfort zones. But if your end goal is to be able to go up a flight of stairs without heavy breathing, to sustain vitality through your older years, then no, do not push. Do just the amount of effort that will allow you to keep coming back, day after day.