Another thing that troubles us about mortality is
the idea that we're not going to have time to do
the things we need to do. Even if we don't know what
that is-- and I'm not sure that anyone really does--
we worry that we won't have time to do it in. (It's like
John Cage said when Life magazine asked him and other
notables "Why Are We Here?"--"No why. Just here.")
Life can't have meaning except what we find in it. We should be nice to each other and do unto others. We should be gentle and kind and forgiving and generous. Patient with others as well as yourself. Besides these ideas, it's up to you to work out your destiny.
There's a good chance that you'll be happier if you're not a crusader, or at least not a crusader all the time. So once you figure out what part of your life you need to devote to the causes are important to you, once you take into account the time you need to spend at work, to keep body and soul together, your time is yours. If you can fill it with exciting adventure, living one big beer commercial of a lifetime, that's good. If you live in simpler circumstances, if you rarely look beyond a night of tv, a few beers, a good book... that can be fine as well, so long as you can be fine with it. (Romantic love help as well; most people can find if it they search, but almost everyone will be stuck without it for some period in their life.)
So figure out what makes you happy, and do it; be content in the fact that you can do things to make you happy, and don't worry that time is wasting or that you don't have forever to waste time in; you have your own lifespan, and that's all anyone will ever have or has ever had.