Aylesbury strikes again

Something in the air disagrees with me - having been fine for weeks, as soon as I go running in Aylesbury, I start wheezing. Not badly (not as bad as at Christmas anyway), but enough to feel uncomfortable. I did the planned session, 45 mins inc. 18 @ threshold, but threshold (160 ish BPM) seemed to relate to a low running speed (presumably because I wasn't getting enough oxygen?). It came out at 4.8 miles total, i.e. about 9:25 per mile average, but with 18 mins steady, 18 quicker, and 9 cool down. Without pushing it ridiculously speed-wise, my max HR was 186, which seems a tad high, but the speed/distance is OK.