With my first baby I went to hospital at 39+5 to be induced due to severe SPD pain... was told i was already 3cm! I had been achey on and off in the back but didn't think too much of it! Unfortunately baby was OP. I didn't have him until 3 days later, when i had been having contractions on and off, then backwaters went. They ARM due to infection risk 48hrs later. Once that kicked off it was about 12 hours.
With baby number two, I was actually very ill, and went into hospital for my own health at 39w. As it turns out I had septicaemia. Baby was becoming distressed so they decided to induce... and realised I was already 2cm. No contractions at that point, just very achey and sore but think that was due to being heavily pregnant in August! They did a rather heavy handed sweep and contractions started. Waters didn't go, so again I had ARM as they wanted to speed things up. I needed the drip in the end as well, but as I said, I was terribly ill and rather weak at this point so not sure if that had something to do with it. Think my notes said active labour was 6 hours. (I had gone to hospital at about 4am and had baby at about 9pm)
It will be interesting to see what happens with this baby. I wonder when i would have "noticed" things were kicking off if the medical professionals hadn't been involved... If i hadn't already been in hospital would i have just carried on as normal and only really gotten contractions later on, at maybe 4/5cm and dealt with the pain etc better? Would things have been more relaxed as they were on my time schedule not the drs? I don't know... I am hoping I find out with this one!
If i were you op i would relax, and be thankful that you're body is already getting ready for active labour without much pain thankfully. I would do my best to keep active, lots of walking etc, and lots of early nights to stock up on sleep whilst you can. Good luck.