a bad long run

on thursday did the usual 5 laps without problem (6 km). Not going out too fast since I had to do the 10 miler on saturday.
On saturday, I was planning on running early, since it was going to be a warm day (22C). But of course, I went out too late on friday, so I only could run around 2pm. I didn't start out too fast, but I was running in the sun the whole time and at one point it really got to me. After doing a big tour of 13km, I was totally exhausted by the heat, and I didn't find the strength to run to the park and do 2-3 laps to finish. So, instead of doing 16km, I only did 13. That's worse then last week since I ran 15k then, and I felt like I could run more. So, it must have been the heat. I wasn't too happy about it. Normally I have an easy long run next week (7 miles), so I was thinking about running the 16k then. When I look at the schedule, I should run 7 miles next week, 12 the next one and 13 the one after that. But in 4 weeks, I have to run the 20k of Brussels. So, maybe it's better if I run 16k next week, then have a set-back week of 7 miles, and then build up again to peek with the 13 miler (20k of Brussels) in weeks. I think that's a good plan.