why is swimming so difficult for me?

[MikeWaters]

I'm reasonably fit. I can do a century on my bike. I play basketball up to 5 times a week.

But after I did the 100 yard swim check yesterday, I felt like I had just run the 400m dash.

I don't think my technique is terrible. I was doing the breast stroke and the sidestroke because it is easier for me to breathe, and I get more tired doing the crawl.

I realize the breaststroke is using muscles I don't normally use all that much, but I don't think that explains it.

One problem that I have is that I don't float. I sink like a stone. To pass the float portion, I basically have to fill my lungs with air and use my hands to tread water as I am on my back (not really floating). If I am having to not only go forward, but also keep myself afloat, is that taking all my extra energy?

When I was young it was very hard for me to tread water for two minutes, while the people next to me were happy as clams not winded at all.

So what's the deal?