Jim-Engr here as well and you are asking the right questions. Good for you.

Based on data I've seen, I suspect your inseam is much longer than the average person's for your height. However, long femurs were a big advantage for distance running as you probably know! But, having the longer legs is going to be trickier to fit you onto a road bike. I would start with the frame size needed for your leg length. However you might need to shorten the stem some to keep your reach within reason. You can get a good Ritchey alloy stem for $70 so you might keep that in mind when looking at used bikes. Avoid going with too long a reach-it can cause all kinds of joint and discomfort issues on longer rides (lessons I've learned the hard way).

One of our LBS's will do a fitting for $70 which can give you the measurements you need to work off of. That might be an option if you have someone around you feel comfortable using. Having their information will take a lot of the guess work out of the selection process plus you can use it as a basis for years to come. But then again many of us Engrs tend to want to research all this ourselves and take the information we learn and just do things ourselves!!

Best of luck regardless of the approach!