Competition in triathlon is super weird, so as a runner that had a tri friend explain it to me I will try to explain it back...
One of the cool things about a road race is everyone is racing on equal footing. In theory, any joe can toe the line against the best in the world and see how they match up. Now you might try to get me on technicalities like chip time/gun time, separate elite starts, etc., but for the most part it's one big race. This is NOT how triathlons are set up.
In Ironman Tris, everyone is in an age bucket and they compete ONLY against people in the same bucket. I think the buckets are divided by gender and every 5 years, you you have 30-34M, 25-29F, so on and so forth. There are only like 5 or 6 Ironmans a year in America (that is, the full distance and sposered by the Ironman people). In addition, there are 10-20ish Half Ironmans. At each Ironman and Half Ironman, the top X spots in EACH BUCKET get invites to Kona. I think X depends on how many competitors there are in the race, and in a given race it varies by bucket.
Now, all that applies ONLY TO AMATEURS. There are two more totally separate divisions in triathon: male pro and female pro. Pros do not get counted in age group results and likewise amateurs CANNOT get counted amoung the pros. So for example, if the top pro runs 3:40 at the Whatever Half Ironman and supertalent Average Joe signs up and does 3:20, he gets jack diddly in the way of prize money and isn't really even considered the winner of the race (I don't think this really ever happens, although it might be the case that the top amateur beats some of the pros that get prize money).
Now, becoming a pro isn't as simple as you might think. You don't just email the race director and say you want to be pro, you have to be a certified pro by (I think) USAT. In order to do that, you have to perform well in a whole bunch of races as an amateur. I guess qualifying for Kona is different for pros and amateurs: amateurs need one good finish at any Ironman. Pros need to accumulate points with a number of high finishes in a certain time period. I assume Lance is a pro and would not want an age group slot, and I have no idea where he stands in qualification.
The last thing I'll say is the only tri I've spectated was a Half Ironman and it had a wave start, so first the pros would go. Then some time later, a whole age bucket would start and then every 5 minutes a new age bucket would go until everyone had started. So literally you were only competing against your bucket because everyone else had different start times. I think some races do have mass starts, although there might be a separate pro start (no idea).