Couple thousand miles+ on my road bike with Tiagra and it's fine. Shifts smooth, rides smooth. I'm glad I chose the bike that felt best over the bike with the "better" component group. The fact that it was cheaper was a bonus. Remember you can always upgrade later and probably for much less than $300 by buying over CL or ebay.
I ride much like you - local charity rides, centuries, etc. I did RAGBRAI on it two years ago, all rolling hills so constant shifting, and it was great.