I would say the most important thing, after politely letting others know that you will be in the lane, is watch what other people are doing and talk to your lane-mates.
When you have designated lanes: slow, medium, and fast, then the choice is somewhat simple. Slow to drill. Medium or fast to swim.
But mostly I find that talking to the people in the lane is best. Let them know that you are going to practice your IM or that you are practicing backstroke.
Also you need to determine if you are going to swim circles or side-to-side. Competitive swimmers will want to swim circles more often, while most fitness swimmers will much prefer side-to-side.
When you start is also important in a shared lane. If you want to swim butterfly, let the other person start first and follow, moving to one-arm at the end of the length. All other strokes, I prefer to start opposite the side of my lane mate while swimming side-to-side. Most people are uncomfortable with someone trailing behind them in the water so to be polite, I try and stay as close to the opposite side as possible. It also makes it much harder for me to catch the person.
However, I think the best lane mates are non-swimmers. The people who are doing aerobics, walker-walking, or floating around in some non-standard way. Like you, they are performing non-traditional endurance swimming and are much less likely to get upset if you drill and swim in their lane.
As with so many things, if you are a regular swimmer at the pool and are polite in how you share a lane, people who see you will invite you into their lane so that they get a good lane-mate.