Have been doing a very steady diet of 5:10 tempo. Recently ran a 3x 1600 in 4:41 off just under 4:00 rest feeling slightly slower than 5000 pace, in very hot weather. Today, in relatively hot and humid weather (85*, 65* dew point with 10 mph wind) I ran 2x 1000 tempo in 3:17, 3:14; 4x400 at probably 1200 effort in 60.3, 59.9, 61.1, 59.8 off 2:00 rest, followed by a couple of marathon-effort reps. The 400s felt very, very controlled and I ran them at an even pace - I split 30 and 45 on all, not coming through in 43 and fading. My official 800 PR is only 1:59 but I believe I was in easily 1:56 high / 1:57 low shape at the very worst as I ran it with a significant negative split.
My previous PR is 4:19 set with suboptimal splits (62, 70, 68, 58 and change). In a race with good competition and a flat, fair course, could I break 4:10?