What they said... but personally I recommend getting a different pack for day hiking than for overnighting.

The heaviest thing you carry day hiking is water and you usually carry all you will need for the day so a hydration pack makes a lot of sense.  Just make sure to dry out the bladder each time you use it.  You don't need to carry that much gear day hiking so a much smaller lighter pack can be used.  You don't necessarily have to have a pack at all.  The main reason is that it conveniently groups all you things together so you can get out the door.  If you known you will hike long, far and often then get fitted for something.  Otherwise it is probably not that critical and you can just get something inexpensive and get hiking.  Later buy something better if you find you need it.

An overnight pack has to be sized to carry the bulk and weight of the gear you are going to use.  There is not much difference between what you take for 1 night vs 2 or 3.  Longer trips require more food and supplies but not necessarily more gear.   Food for a couple of days is not the bulkiest thing you will carry although it can add significant weight.  For such a pack I strongly recommend you first pay attention to the weight and bulk of the gear you will carry and then get fitted for a pack that can carry the weight and bulk of that gear.  If your budget means that you intend to use some existing car camping gear you have, take note that it tends to be heavier and bulkier.  For the first modern pack I bought I had to exchange a 55L for a 70L for that reason.  The pack I bought comfortably carries 50# or more and was quite expensive.  I still have it and have used it for overnights and week long section hikes.

Triple Crowners carry relatively little gear btw.  The goal is to travel light and fast which you can do on well established trails where you can re-supply frequently.  They often get away with small packs because of that so that style of hiking is not always the best example if your intent is shorter trips to hang out at the lake where you may want to take more gear.