The risk is that the blisters will get worse and worse, and continue to interfere with your hiking experience. They can get larger, more painful, and eventually tear open and risk getting infected. This process can happen fairly quickly - from the first time you notice pain in your feet, blisters can develop in < 15 minutes. I haven't personally tried to hike on them longer than that, but I don't think it would take very long (a single day, if you have enough rubbing) for a blister to rupture. Once you have an open wound, infection becomes a continual risk unless you can keep it clean and dry, which is hard to do with your feet while you're hiking. (And, to address your question, all infections are very serious.)
Long distance walking
You mentioned a 30km hike... I'd recommend that you start with short hikes, and add distance slowly over a period of weeks or months. This will give you a chance to:
- Find a set of socks and shoes (possibly including "liner socks") that work for you.
- Break those shoes in.
- Develop first hand experience identifying and treating blisters (with moleskin or duct tape) during the hike.
- Toughen up your feet gradually.
- Get in shape (if you weren't already).
I don't know that you were specifically planning this, but going "off the couch" to a 30km hike without any other preparation is setting you up for a variety of overuse injuries (in addition to blisters, you could develop knee or ankle problems) that could be avoided by a more gradual training experience.