Spend zero time worrying about building your personal brand. Spend all your time focused on achieving your goals building your business. Simply put, the best way to build your brand is to achieve success. Let others evangelize you, whether it be investors impressed with your growth, customers who are delighted by the product/service you provide, or even your employees talking about what a great leader you are and what an amazing work environment you create or how brilliant you are and how you're revolutionizing the industry you're operating in.
These are all so much more important than the attention you could get by public speaking, blogging, doing interviews, etc.
A Company can be artificially inflated by someone able to create attention but it is not a sustainable path to success.
Others singing your praises is so much more substantive than promoting yourself.
Having a great "personal brand" with loads of Twitter followers and a roster of conference speaking appearances pales in comparison to having legitimate, close connections to people who trust you and will vouch for you in any situation. Who are those people in your life? Who do they know whom you want to know? Ask for some introductions.
Frankly, a lot of social media and personal brand consultants are full of crap when they tell you to promote-promote-promote at all costs. When you're out giving keynote after keynote after keynote, you aren't building your product. Worse, it can also result in a startup brand that can't stand alone without its CEO, and for the long-term health of the company, you definitely don't want that.
Something you're going to want to consider is that while you don't like to be in the limelight, you may need to use some "limelight" tactics to build your company's reputation. I've seen a lot of successful cases in which a reserved or introverted entrepreneur makes sure that he/she has a reputable senior exec at the company -- maybe a CMO or marketing VP -- to be more "out there." You don't have to promote yourself, but somebody has to be promoting what you do. A respected advisor to your company can do this too in some cases.
Interview fellow experts. Do google hangouts with them. Audio calls. Post these in forums to get engagement. A thread with 20 replies and 250 views has a ton more credibility than a lonely blog post on your site. You are who you hang around. (Get 4 or 5 of these together and you can sell them as an information product!)
Get on radio and podcast interviews. Market these. They position you as the expert.
Publish a Kindle and/or Createspace book. This doesn't have to be lengthy; just answer a specific question in your field. Again, positioning. It's there...even if few people buy it. You can market it in your LinkedIn profile, your site, etc. and it boosts credibility just by being there.
Before building your credibility, make sure you accomplish some goals so that you gain experience from them. Failures are great since they teach you a lot.
From there, build your online personal presence so that if anyone searches you on Linkedin or Google, they can find some information about you.
Then, join many groups online either on Linkedin, Facebook or other websites. From there, try helping people within the community that could benefit from your expertise.
This is how I did it myself and it works great. Baby steps do help!
Hope this helps.
I hate to break it to you but you ARE your brand, especially in this day and age. Go join Toastmasters and get out of your protective shell. Take pride in what you do and see it as a calling. Once you get over yourself, there's no turning back. We're all human and no one is perfect. Stop putting pressure on yourself to be perfect and you'll do just fine in front of an audience.