Update: I ran really slow, like 9-minute miles. Part of this was due to the heat: I have been used to 20-30F and it was in the mid to high 60's. The other part was that this is unfamiliar countryside for me and I didn't want to push myself to the edge of death on a run where there was a good chance of getting lost--and thus not finishing at the expected time.
Today I ran in the cold, it was snowing and there was slick ground, but I still ran much faster than last Saturday. I need to get acclimated to the heat since late April can be pretty hot here and I have a marathon of reasons for wanting to be prepared.