I did my first swim/bike brick yesterday; this is a swim workout followed by a bike ride. I did a pretty intense swim. My main set was 400m, 200m, 100m, and 50m at my fastest pace (albeit with a lot of rest in between). I haven't swum that fast for a long time.
I had a few moments of "flow", the semi-nirvana state of everything working right; my stroke felt fast and efficient and my turns were crisp. Inevitably, my arms would get tired and my lungs would ache, disappating the "flow". I'm still a lot slower than I was during my high school swimming days, still, the swim felt good.
When I went to the locker room to change for the biking, I found myself a little dazed. I made a bunch of big mistakes like putting my biking pants on backwards and then my shirt on inside-out. I had a hard time concentrating. I had read about the difficulties people have during their transitions in triathlons, but this was my first real experience.
Once I got on the bike (a stationery spinning bike, actually), I found my legs didn't work quite right and everything seemed harder than normal. I don't normally start my rides already tired, so I suppose this is to be expected. After a few minutes, things settled in, and I felt OK. Watching TV (Seahawks game highlights -- go Hawks! -- and the first part of Total Recall) helped pass the time. I threw in a bit of weight lifting afterwards for good measure.
I wasn't sure I could swim for 30min and ride for 60 so it felt good to finish it without feeling like I needed to die. Those will be longer than the times I need to sustain for a sprint triathlon. I just need to be able to run 5K after doing the swim and bike... And I definitely need to practice transitions.