swim - when I don't swim for a few weeks, I forget how much I purely like it and start dreading how awful it will be to get back in the pool, but in fact I just resolutely ignored the clock and found that after a couple hundred yards, I felt completely comfortable (even strong) in the water. Swimming does not give me as much of an endorphin rush as running, but I feel immediately good as soon as I have swum even a length, it's pretty amazing...
I am currently finding it works well to break the workout into 4 x 600, for mental variety:
3 x (100 free, 100 drill): (1) right arm, left arm, catch-up, full stroke; (2) fists/swim by 25 (3) front scull/swim by 25
8 x 75 kick-drill-swim, 2 of each stroke in IM order
2 x 100 IM kick, 100 IM drill, 100 IM
3 x (4 x 50 as stroke down, free back), first set fly, second set back, last set breast
2400 yards total
plus 10 miles bike - it is a ferociously windy day out there, low 40s - on the way back I was slightly afraid my bike was actually going to be blown over, but as I was on the bike path this was exhilarating rather than scary!