Well, yes, it's important to get a bike model and size that is appropriately-sized, including your reach to the handlebars. If you are short of leg and long of torso, you will probably need the very tallest frame you can just barely straddle, and you may also need to swap from the stock handlebar stem to a longer one. Upright is comfortable for riding to the corner store, or for just sitting on the bike watching TV, but it's not the right position for cycling any distance. You want to lean forward so you can make use of all the muscles involved in pedalling, and also to better distribute your weight between your hands and your behind.