I'm not sure if it was the heat today or if I really am even more unfit than I thought, but I did find it a struggle despite my slow pace.

!LOL same here! After the winter which has been unrelentingly cold, windy and wet... the heat seems to have come out of nowhere! I'm not complaining too much as it is long overdue, but I do feel like I'm trying to acclimatize to it!