Used is a great way to go (I used to be a distance runner on a budget too).
Browse your local Craigslist a bit. You can pick up a nice lightweight early-90s steel road bike with STI shifters (as opposed to downtube shifters) for around $300, sometimes less. In my case, I found a beautiful Performance brand road bike in my size, beautiful condition, with full Shimano 105 including STI shifters all for $250. The bike looks modern, feels and rides modern, and really isn't far at all from what you'll find in the bike shop these days.
So $750 - 250.. that leaves you $500. Spend $100 of that on accessories, and pocket the remaining $400.