On my first 2 rides..
I knew I was unfit but bloody hell, I didn't realise just how unfit. I thought I'd be able to do around 5 miles.. not a chance! Managed 2.4 miles today! TBH I'm not getting out of breath or tired.. it's like my legs just give up and turn to lead.
As one of the bigger miler munchers on MCL as kindly mentioned by avsd.
7 years ago when I took up cycling after a 20 year break. My first commute was 1 and a half miles. Easy on the way there - all down hill. On the way back it took 7 minutes to ride back and over half and hour to recover, I had to go have a lie down. My wife said it looked like I was going to have a heart attack.
This story is not exclusive to me on Cycle Chat.
There are many on here that have similar stories to tell.
So..keep it up, take it easy and build it up slowly but surely.
You will get better, faster and fitter. Guaranteed.