Spiney The class i started in Easter 2 years ago had and still has some complete beginners. I spent some time in the basic level and then went up to intermediate.
I could swim a length initially when I moved up, then 2, then 3.
I wrote something similar last night on the zero to mile group. You might find this of use:
When I was making the transition between lengths, (only been swimming 2 years) to get to that 50m I would swim a couple of 25m until I was warmed up a bit, then I would tell myself that I was going to try to swim 50m, so would push off and swim really slowly to get to the end of the pool, then I would start on the 2nd length.
Just keep that slow and steady pace and see how far you get. You should be able to make a 1/3 of a length without any effort at all.
If you struggle there, flip over on to your back and do back stroke to the end.
Recover a little and try it again. See if you can get half way there.
Then push on and do the last half.
The main thing is not to start off too fast, keep slow and steady and you will get there.
I was you 2 years ago.
A year ago I was swimming 50m chunks, 6m ago I was capable of 100m chunks if I prepared my self for it.
Today I did 1800m pretty much continuous - albeit Total Immersion, but full conventional stroke wouldnt have made that big a difference i don't feel
You absolutely can do this! If I can, you definitely can.
Please keep posting, asking and let us know how you get on, any questions etc.. we'll get you belting up and down the pool in no time!