You can only put one mission in each cabin.
I think you have the same cabins as me. I regularly get over 20 mil per run, but if you go too fast, the missions dry up, so I'm averaging about 75 mil per hour, which is what you can expect after ranking up. After about 4 20-22 mil runs, the missions seem to dry up, so I make a cup of tea, and jump on the forum for 15 minutes, then when i go back, more missions are listed.
Your hull is paper thin. If you touch anything, you'll lose some of it. When you drop out at the beacon, make sure that you hit your zero throttle button or key before you bring up the galaxy map. If you forget, your ship will crash into the beacon and you'll lose so much hull that some of the missions will fail. The Belugas also have a nasty habit of flying into you while you have the galaxy map open, so have a quick look to see whether any are on a collision course before you open the map. Always use the docking computer at the outpost to land the ship without damage, but only for the very last bit. Don't be lazy and engage it as soon as you have docking permission otherwise you might get scanned. Finally, be careful when you lift off. It's easy to clip the structure. If you do that when you boost away, it'll knock your hull down so far that missions will fail.
Keep an eye on all the comms messages. if you get a "target acquired..." or similar, the NPC will drop out soon after you if it doesn't interdict you, also sometimes even if you win the interdiction. You will be OK as long as you don't hang about at the beacon or landing at the outpost. You can do 10 runs and get no inderdictions, which means that they can take you by surprise when you're not concentrating. If any missions fail, you can expect to see NPCs in the next run.