I'm still kind of in the dark. Are you saying that during a marathon there will be some "sacrificial" runners (I think they're called "rabbits")? That's like playing in a team, they're not really opponents.
You can't interact with the real opponents, as far as I know.
2. Different runners have different strengths. Some have a stronger kick at the end of the race, some have a weaker kick but can maintain a higher pace.
One example of how this can play out in a marathon (and, to a greater extent, in something like the 5000) is that a runner who has a faster overall potential but a weaker finishing kick wants the pace to be run at a higher tempo. However, if the runner tries to do this unilaterally, weaker athletes may be able to draft and use the physical and mental benefits to hang on, before winning using their kick.
However, if there are multiple endurance oriented athletes, they're typically more able to enforce a higher tempo throughout the run that will drop those athletes that have a stronger kick.
You'll often notice this in the 5000, where the winning time might be well below the personal or seasonal bests of many of the participating runners. Tactics prevent them from running their optimal race, and then a stronger kicking runner wins.
rabbits are generally used for time trials/record attempts. if a runner wanted to try for a 4 minute mile, they would have a rabbit run 2 or 3 laps evenly, at slightly better than 4 minute pace (say 800 in 1:58 or 1200 in 2:57). this allows the other racers to draft off the rabbit, as well as focus on running even splits so they have as much energy as possible for the final sprints.
rabbits in marathons are pretty rare (to my knowledge?) though
You can't interact with the real opponents, as far as I know.