Pumpkin problem

im not normally a veg grower so when my friend gave me a pumpkin plant for my birthday in June I thought I'd give it a go. as my garden is fully patiod I bought a decent size trough planter as I knew it would grow quite large. I was assured it would only grow as large as the pot allowed. However, I have had to rearrange my garden 3 times as it is now at least 4 meters long with leaves the size of kids umbrellas!! Lots of males flowers and a few female ones too ( I have done a little research) but it's still growing!!! Can anyone tell me when the pumpkins will start forming and if I trimmed it back would it harm the plant?? many thanks in advance