Rigid fork and slicks will make a big difference, so will some bar ends or trekking bars for more hand positions. If you're made of money, going ahead and getting a touring bike might be an option, but despite what a lot of people on this forum will tell you, there are a lot of people who tour on MTBs and prefer 26" tires for their durability. Seems to be more popular in Europe, but plenty of people do it here.
While I'm not assuming you're poor, I'm assuming you don't want to spend $1400 for a Trek 520 and another $500 on racks, lights, fenders, panniers for something you might end up hating, so for now, a modified mountain bike might be an excellent choice. Heck, you might be like me and find you prefer one.