Looks like a good summer of running for you FM!
If you've never done trail runs, they are a good bit tougher for me than road running. My last 5k trail race was about 4 minutes slower than my 5k road time. Plus they cause more aerobic wear on me. Doing an 18k trail run two weeks after a half marathon might be a bit rough if it is your first half marathon. My wife felt a bit worn out for a bit after her half. I was ok myself later the next week. Probably each person is different.
I'm not sure what a 19km stairs challenge is, but that sounds insanely scary fun
For myself, planned to run 9miles today, but my stomach is super nauseous. Not sure what is going on with it, but I'm hoping it goes away soon or I may try to run through it as much of a disaster that might cause.